diff --git a/backend/mpc-system/services/service-party-app/electron/main.ts b/backend/mpc-system/services/service-party-app/electron/main.ts
index 5fd6c471..a760ac9a 100644
--- a/backend/mpc-system/services/service-party-app/electron/main.ts
+++ b/backend/mpc-system/services/service-party-app/electron/main.ts
@@ -821,22 +821,44 @@ async function handleCoSignStart(event: {
   // 标记签名开始
   signInProgressSessionId = event.sessionId;
 
-  // 从 event.selectedParties 更新参与者列表（类似 keygen 的 handleSessionStart）
-  // 这确保我们使用的是实际加入会话的参与者，而不是预设的列表
+  // 打印当前 activeCoSignSession.participants 状态
+  console.log('[CO-SIGN] Current activeCoSignSession.participants before update:',
+    activeCoSignSession.participants.map(p => ({
+      partyId: p.partyId.substring(0, 8),
+      partyIndex: p.partyIndex,
+    }))
+  );
+  console.log('[CO-SIGN] event.selectedParties:', event.selectedParties?.map(id => id.substring(0, 8)));
+
+  // 从 event.selectedParties 更新参与者列表
+  // 优先使用 activeCoSignSession.participants 中的 partyIndex（来自 signingParties 或 other_parties）
   if (event.selectedParties && event.selectedParties.length > 0) {
     const myPartyId = grpcClient?.getPartyId();
     const updatedParticipants: Array<{ partyId: string; partyIndex: number; name: string }> = [];
 
-    event.selectedParties.forEach((partyId, index) => {
+    event.selectedParties.forEach((partyId) => {
       // 查找已有的参与者信息
       const existingParticipant = activeCoSignSession?.participants.find(p => p.partyId === partyId);
-      updatedParticipants.push({
-        partyId: partyId,
-        partyIndex: existingParticipant?.partyIndex ?? (index + 1),
-        name: partyId === myPartyId ? '我' : (existingParticipant?.name || `参与方 ${index + 1}`),
-      });
+      if (existingParticipant) {
+        // 使用已有的 partyIndex
+        updatedParticipants.push({
+          partyId: partyId,
+          partyIndex: existingParticipant.partyIndex,
+          name: partyId === myPartyId ? '我' : existingParticipant.name,
+        });
+      } else {
+        // 找不到已有信息，这不应该发生 - 记录警告
+        console.warn(`[CO-SIGN] WARNING: Party ${partyId.substring(0, 8)} not found in activeCoSignSession.participants!`);
+        // 不使用 fallback index，直接跳过，这会导致参与者数量不足，签名会失败
+        // 这样可以更早发现问题
+      }
     });
 
+    // 检查参与者数量是否符合预期
+    if (updatedParticipants.length !== event.selectedParties.length) {
+      console.error(`[CO-SIGN] ERROR: Participant count mismatch! Expected ${event.selectedParties.length}, got ${updatedParticipants.length}`);
+    }
+
     // 按 partyIndex 排序
     updatedParticipants.sort((a, b) => a.partyIndex - b.partyIndex);
 
@@ -1651,15 +1673,17 @@ function setupIpcHandlers() {
 
         if (joinResult?.success) {
           // 设置活跃的 Co-Sign 会话信息
-          // 注意：这里只初始化发起者自己，其他参与者会在 session_started 事件中从 selectedParties 更新
-          const signParticipants: Array<{ partyId: string; partyIndex: number; name: string }> = [];
+          // 使用 signingParties 初始化完整的参与者列表（包含正确的 partyIndex）
+          const signParticipants: Array<{ partyId: string; partyIndex: number; name: string }> = signingParties.map(p => ({
+            partyId: p.party_id,
+            partyIndex: p.party_index,
+            name: p.party_id === partyId ? (params.initiatorName || '发起者') : `参与方 ${p.party_index + 1}`,
+          }));
 
-          // 添加发起方
-          signParticipants.push({
-            partyId: partyId,
-            partyIndex: joinResult.party_index,
-            name: params.initiatorName || '发起者',
-          });
+          console.log('[CO-SIGN] Initiator signParticipants (from signingParties):', signParticipants.map(p => ({
+            partyId: p.partyId.substring(0, 8),
+            partyIndex: p.partyIndex,
+          })));
 
           activeCoSignSession = {
             sessionId: result.session_id,
@@ -1768,6 +1792,7 @@ function setupIpcHandlers() {
     walletName?: string;
     messageHash: string;
     threshold: { t: number; n: number };
+    parties?: Array<{ party_id: string; party_index: number }>;
   }) => {
     try {
       const partyId = grpcClient?.getPartyId();
@@ -1786,18 +1811,37 @@ function setupIpcHandlers() {
       const result = await grpcClient?.joinSession(params.sessionId, partyId, params.joinToken);
       if (result?.success) {
         // 设置活跃的 Co-Sign 会话
-        const participants: Array<{ partyId: string; partyIndex: number; name: string }> = result.other_parties?.map((p: { party_id: string; party_index: number }, idx: number) => ({
-          partyId: p.party_id,
-          partyIndex: p.party_index,
-          name: `参与方 ${idx + 1}`,
-        })) || [];
+        // 优先使用 params.parties（来自 validateInviteCode，包含所有预期参与者）
+        // 而不是 result.other_parties（只包含已加入的参与者）
+        let participants: Array<{ partyId: string; partyIndex: number; name: string }>;
 
-        // 添加自己
-        participants.push({
-          partyId: partyId,
-          partyIndex: result.party_index,
-          name: '我',
-        });
+        if (params.parties && params.parties.length > 0) {
+          // 使用完整的 parties 列表
+          participants = params.parties.map(p => ({
+            partyId: p.party_id,
+            partyIndex: p.party_index,
+            name: p.party_id === partyId ? '我' : `参与方 ${p.party_index + 1}`,
+          }));
+          console.log('[CO-SIGN] Participant using params.parties (complete list):', participants.map(p => ({
+            partyId: p.partyId.substring(0, 8),
+            partyIndex: p.partyIndex,
+          })));
+        } else {
+          // Fallback: 使用 other_parties + 自己（可能不完整）
+          console.warn('[CO-SIGN] WARNING: params.parties not available, using other_parties (may be incomplete)');
+          participants = result.other_parties?.map((p: { party_id: string; party_index: number }, idx: number) => ({
+            partyId: p.party_id,
+            partyIndex: p.party_index,
+            name: `参与方 ${idx + 1}`,
+          })) || [];
+
+          // 添加自己
+          participants.push({
+            partyId: partyId,
+            partyIndex: result.party_index,
+            name: '我',
+          });
+        }
 
         // 按 partyIndex 排序
         participants.sort((a, b) => a.partyIndex - b.partyIndex);
diff --git a/backend/mpc-system/services/service-party-app/src/pages/CoSignJoin.tsx b/backend/mpc-system/services/service-party-app/src/pages/CoSignJoin.tsx
index c7636126..8e3525e0 100644
--- a/backend/mpc-system/services/service-party-app/src/pages/CoSignJoin.tsx
+++ b/backend/mpc-system/services/service-party-app/src/pages/CoSignJoin.tsx
@@ -10,6 +10,11 @@ interface Share {
   threshold: { t: number; n: number };
 }
 
+interface SignPartyInfo {
+  party_id: string;
+  party_index: number;
+}
+
 interface SessionInfo {
   sessionId: string;
   keygenSessionId: string;
@@ -18,6 +23,7 @@ interface SessionInfo {
   threshold: { t: number; n: number };
   status: string;
   currentParticipants: number;
+  parties?: SignPartyInfo[];
 }
 
 interface ValidateResult {
@@ -202,6 +208,7 @@ export default function CoSignJoin() {
       walletName: sessionInfo.walletName,
       messageHash: sessionInfo.messageHash,
       threshold: sessionInfo.threshold,
+      parties: sessionInfo.parties,
     });
 
     try {
@@ -213,6 +220,7 @@ export default function CoSignJoin() {
         walletName: sessionInfo.walletName,
         messageHash: sessionInfo.messageHash,
         threshold: sessionInfo.threshold,
+        parties: sessionInfo.parties,
       });
       console.log('[CoSignJoin] joinSession result:', result);
 
diff --git a/backend/mpc-system/services/service-party-app/src/types/electron.d.ts b/backend/mpc-system/services/service-party-app/src/types/electron.d.ts
index fe5fc64d..85d57ae0 100644
--- a/backend/mpc-system/services/service-party-app/src/types/electron.d.ts
+++ b/backend/mpc-system/services/service-party-app/src/types/electron.d.ts
@@ -456,6 +456,7 @@ interface JoinCoSignSessionParams {
   walletName?: string;
   messageHash: string;
   threshold: { t: number; n: number };
+  parties?: Array<{ party_id: string; party_index: number }>;
 }
 
 interface JoinCoSignSessionResult {