diff --git a/backend/mpc-system/services/service-party-app/src/pages/CoSignSession.tsx b/backend/mpc-system/services/service-party-app/src/pages/CoSignSession.tsx
index f1ae52d7..db54abcf 100644
--- a/backend/mpc-system/services/service-party-app/src/pages/CoSignSession.tsx
+++ b/backend/mpc-system/services/service-party-app/src/pages/CoSignSession.tsx
@@ -130,11 +130,10 @@ export default function CoSignSession() {
       if (storedTxInfo) {
         try {
           const parsed = JSON.parse(storedTxInfo);
-          // 恢复 bigint 类型
+          // 恢复 bigint 类型 (Legacy 交易使用 gasPrice)
           if (parsed.preparedTx) {
             parsed.preparedTx.gasLimit = BigInt(parsed.preparedTx.gasLimit);
-            parsed.preparedTx.maxFeePerGas = BigInt(parsed.preparedTx.maxFeePerGas);
-            parsed.preparedTx.maxPriorityFeePerGas = BigInt(parsed.preparedTx.maxPriorityFeePerGas);
+            parsed.preparedTx.gasPrice = BigInt(parsed.preparedTx.gasPrice);
             parsed.preparedTx.value = BigInt(parsed.preparedTx.value);
           }
           setTxInfo(parsed);
diff --git a/backend/mpc-system/services/service-party-app/src/pages/Home.tsx b/backend/mpc-system/services/service-party-app/src/pages/Home.tsx
index 24f0c8d3..ed72ee0a 100644
--- a/backend/mpc-system/services/service-party-app/src/pages/Home.tsx
+++ b/backend/mpc-system/services/service-party-app/src/pages/Home.tsx
@@ -293,8 +293,7 @@ export default function Home() {
           preparedTx: {
             ...preparedTx,
             gasLimit: preparedTx.gasLimit.toString(),
-            maxFeePerGas: preparedTx.maxFeePerGas.toString(),
-            maxPriorityFeePerGas: preparedTx.maxPriorityFeePerGas.toString(),
+            gasPrice: preparedTx.gasPrice.toString(),
             value: preparedTx.value.toString(),
           },
           to: transferTo,
diff --git a/backend/mpc-system/services/service-party-app/src/utils/transaction.ts b/backend/mpc-system/services/service-party-app/src/utils/transaction.ts
index 714b99ca..d11483e9 100644
--- a/backend/mpc-system/services/service-party-app/src/utils/transaction.ts
+++ b/backend/mpc-system/services/service-party-app/src/utils/transaction.ts
@@ -50,21 +50,24 @@ export interface TransactionParams {
 
 /**
  * 准备好的交易 (包含所有必要字段)
+ * 使用 Legacy 交易格式 (Type 0)，因为 KAVA 不支持 EIP-1559
  */
 export interface PreparedTransaction {
   chainId: number;
   nonce: number;
-  maxPriorityFeePerGas: bigint;
-  maxFeePerGas: bigint;
+  gasPrice: bigint;  // Legacy 使用 gasPrice 而不是 maxFeePerGas
   gasLimit: bigint;
   to: string;
   value: bigint;
   data: string;
-  accessList: unknown[];
   // 用于签名的哈希
   signHash: string;
   // 原始交易数据 (用于签名后广播)
   rawTxForSigning: string;
+  // 为了兼容性保留这些字段
+  maxPriorityFeePerGas?: bigint;
+  maxFeePerGas?: bigint;
+  accessList?: unknown[];
 }
 
 /**
@@ -392,7 +395,8 @@ export async function estimateGas(params: { from: string; to: string; value: str
 }
 
 /**
- * 准备 EIP-1559 交易
+ * 准备 Legacy 交易 (Type 0)
+ * KAVA 不支持 EIP-1559，所以使用 Legacy 格式
  * 返回交易数据和待签名的哈希
  */
 export async function prepareTransaction(params: TransactionParams): Promise<PreparedTransaction> {
@@ -400,9 +404,8 @@ export async function prepareTransaction(params: TransactionParams): Promise<Pre
 
   // 获取或使用提供的参数
   const nonce = params.nonce ?? await getNonce(params.from);
-  const gasPrice = await getGasPrice();
-  const maxFeePerGas = params.maxFeePerGas ?? gasPrice.maxFeePerGas;
-  const maxPriorityFeePerGas = params.maxPriorityFeePerGas ?? gasPrice.maxPriorityFeePerGas;
+  const gasPriceData = await getGasPrice();
+  const gasPrice = params.maxFeePerGas ?? gasPriceData.maxFeePerGas;
   const gasLimit = params.gasLimit ?? await estimateGas({
     from: params.from,
     to: params.to,
@@ -412,81 +415,72 @@ export async function prepareTransaction(params: TransactionParams): Promise<Pre
 
   const value = kavaToWei(params.value);
   const data = params.data || '0x';
-  const accessList: unknown[] = [];
 
-  // EIP-1559 交易字段顺序
-  // [chainId, nonce, maxPriorityFeePerGas, maxFeePerGas, gasLimit, to, value, data, accessList]
+  // Legacy 交易字段顺序 (EIP-155)
+  // [nonce, gasPrice, gasLimit, to, value, data, chainId, 0, 0]
+  // 最后三个字段用于 EIP-155 replay protection
   const txFields = [
-    chainId,
     nonce,
-    maxPriorityFeePerGas,
-    maxFeePerGas,
+    gasPrice,
     gasLimit,
     params.to.toLowerCase(),
     value,
     data,
-    accessList,
+    chainId,
+    0,  // EIP-155: v placeholder
+    0,  // EIP-155: r placeholder
   ];
 
   // RLP 编码交易字段
   const encodedTx = rlpEncode(txFields);
 
-  // EIP-1559 交易类型前缀: 0x02
-  const txWithType = new Uint8Array(1 + encodedTx.length);
-  txWithType[0] = 0x02;
-  txWithType.set(encodedTx, 1);
-
-  // 计算签名哈希
-  const signHashBytes = await keccak256(txWithType);
+  // 计算签名哈希 (不需要类型前缀，Legacy 交易直接哈希)
+  const signHashBytes = await keccak256(encodedTx);
   const signHash = bytesToHex(signHashBytes);
 
   return {
     chainId,
     nonce,
-    maxPriorityFeePerGas,
-    maxFeePerGas,
+    gasPrice,
     gasLimit,
     to: params.to.toLowerCase(),
     value,
     data,
-    accessList,
     signHash,
-    rawTxForSigning: bytesToHex(txWithType),
+    rawTxForSigning: bytesToHex(encodedTx),
   };
 }
 
 /**
  * 使用签名完成交易并返回可广播的原始交易
+ * 使用 Legacy 交易格式 (Type 0)
  */
 export function finalizeTransaction(
   preparedTx: PreparedTransaction,
   signature: { r: string; s: string; v: number }
 ): string {
-  // EIP-1559 签名后的交易字段
-  // [chainId, nonce, maxPriorityFeePerGas, maxFeePerGas, gasLimit, to, value, data, accessList, v, r, s]
+  // EIP-155: v = chainId * 2 + 35 + recovery_id
+  // recovery_id 是 0 或 1
+  const v = preparedTx.chainId * 2 + 35 + signature.v;
+
+  // Legacy 签名后的交易字段
+  // [nonce, gasPrice, gasLimit, to, value, data, v, r, s]
   const signedTxFields = [
-    preparedTx.chainId,
     preparedTx.nonce,
-    preparedTx.maxPriorityFeePerGas,
-    preparedTx.maxFeePerGas,
+    preparedTx.gasPrice,
     preparedTx.gasLimit,
     preparedTx.to,
     preparedTx.value,
     preparedTx.data,
-    preparedTx.accessList,
-    signature.v, // recovery id (0 or 1 for EIP-1559)
+    v,
     '0x' + signature.r,
     '0x' + signature.s,
   ];
 
   const encodedSignedTx = rlpEncode(signedTxFields);
 
-  // 添加类型前缀
-  const signedTxWithType = new Uint8Array(1 + encodedSignedTx.length);
-  signedTxWithType[0] = 0x02;
-  signedTxWithType.set(encodedSignedTx, 1);
-
-  return '0x' + bytesToHex(signedTxWithType);
+  // Legacy 交易不需要类型前缀
+  return '0x' + bytesToHex(encodedSignedTx);
 }
 
 /**