diff --git a/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt b/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
index cdd7fbf3..c1abe192 100644
--- a/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
+++ b/backend/mpc-system/services/service-party-android/app/src/main/java/com/durian/tssparty/presentation/viewmodel/MainViewModel.kt
@@ -373,51 +373,55 @@ class MainViewModel @Inject constructor(
                     }
                 }
                 "party_joined", "participant_joined" -> {
-                    android.util.Log.d("MainViewModel", "Processing participant_joined event...")
+                    /**
+                     * 【架构安全修复 - 防止参与者计数竞态条件】
+                     *
+                     * 原问题：使用 current.size + 1 递增计数器存在多个风险
+                     * 1. 事件重放：重连后事件可能重复发送，导致参与者重复添加
+                     * 2. 事件乱序：网络延迟可能导致事件乱序到达，参与者编号错乱
+                     * 3. 状态不一致：本地计数与服务端真实参与者列表不同步
+                     *
+                     * 修复方案：使用事件的 selectedParties 字段构建权威的参与者列表
+                     * - selectedParties 来自服务端，是参与者的唯一真实来源
+                     * - 根据 selectedParties.size 构建参与者列表，确保与服务端一致
+                     * - 防止重复添加和计数错乱
+                     */
+                    android.util.Log.d("MainViewModel", "Processing participant_joined event (selectedParties=${event.selectedParties.size})...")
+
+                    // Build participant list from authoritative server data
+                    val participantCount = event.selectedParties.size
+                    val participantList = List(participantCount) { index -> "参与方 ${index + 1}" }
+                    android.util.Log.d("MainViewModel", "  → Building participant list from server data: $participantList")
 
                     // Update participant count for initiator's CreateWallet screen
                     val currentSessionId = _currentSessionId.value
                     android.util.Log.d("MainViewModel", "  Checking for initiator: currentSessionId=$currentSessionId, eventSessionId=${event.sessionId}")
                     if (currentSessionId != null && event.sessionId == currentSessionId) {
-                        android.util.Log.d("MainViewModel", "  → Matched initiator session! Updating _sessionParticipants")
-                        _sessionParticipants.update { current ->
-                            val newParticipant = "参与方 ${current.size + 1}"
-                            android.util.Log.d("MainViewModel", "  → Adding participant: $newParticipant, total now: ${current.size + 1}")
-                            current + newParticipant
-                        }
+                        android.util.Log.d("MainViewModel", "  → Matched initiator session! Updating _sessionParticipants to: $participantList")
+                        _sessionParticipants.value = participantList
                     }
 
                     // Update participant count for keygen joiner's JoinKeygen screen
                     val joinKeygenInfo = pendingJoinKeygenInfo
                     android.util.Log.d("MainViewModel", "  Checking for joiner: joinKeygenInfo?.sessionId=${joinKeygenInfo?.sessionId}")
                     if (joinKeygenInfo != null && event.sessionId == joinKeygenInfo.sessionId) {
-                        android.util.Log.d("MainViewModel", "  → Matched joiner session! Updating _joinKeygenParticipants")
-                        _joinKeygenParticipants.update { current ->
-                            val newParticipant = "参与方 ${current.size + 1}"
-                            current + newParticipant
-                        }
+                        android.util.Log.d("MainViewModel", "  → Matched joiner session! Updating _joinKeygenParticipants to: $participantList")
+                        _joinKeygenParticipants.value = participantList
                     }
 
                     // Update participant count for sign joiner's CoSign screen
                     val joinSignInfo = pendingJoinSignInfo
                     if (joinSignInfo != null && event.sessionId == joinSignInfo.sessionId) {
-                        android.util.Log.d("MainViewModel", "  → Matched sign joiner session! Updating _coSignParticipants")
-                        _coSignParticipants.update { current ->
-                            val newParticipant = "参与方 ${current.size + 1}"
-                            current + newParticipant
-                        }
+                        android.util.Log.d("MainViewModel", "  → Matched sign joiner session! Updating _coSignParticipants to: $participantList")
+                        _coSignParticipants.value = participantList
                     }
 
                     // Update participant count for sign initiator's TransferScreen (SigningScreen)
                     val signSessionId = _signSessionId.value
                     android.util.Log.d("MainViewModel", "  Checking for sign initiator: signSessionId=$signSessionId, eventSessionId=${event.sessionId}")
                     if (signSessionId != null && event.sessionId == signSessionId) {
-                        android.util.Log.d("MainViewModel", "  → Matched sign initiator session! Updating _signParticipants")
-                        _signParticipants.update { current ->
-                            val newParticipant = "参与方 ${current.size + 1}"
-                            android.util.Log.d("MainViewModel", "  → Adding participant: $newParticipant, total now: ${current.size + 1}")
-                            current + newParticipant
-                        }
+                        android.util.Log.d("MainViewModel", "  → Matched sign initiator session! Updating _signParticipants to: $participantList")
+                        _signParticipants.value = participantList
                     }
                 }
                 "all_joined" -> {