diff --git a/escheduler-dao/src/main/java/cn/escheduler/dao/ProcessDao.java b/escheduler-dao/src/main/java/cn/escheduler/dao/ProcessDao.java
index 38dce40f81..cf217c95aa 100644
--- a/escheduler-dao/src/main/java/cn/escheduler/dao/ProcessDao.java
+++ b/escheduler-dao/src/main/java/cn/escheduler/dao/ProcessDao.java
@@ -120,48 +120,64 @@ public class ProcessDao extends AbstractBaseDao {
      * find one command from command queue, construct process instance
      * @param logger
      * @param host
-     * @param vaildThreadNum
+     * @param validThreadNum
      * @return
      */
     @Transactional(value = "TransactionManager",rollbackFor = Exception.class)
-    public ProcessInstance scanCommand(Logger logger, String host, int vaildThreadNum){
+    public ProcessInstance scanCommand(Logger logger, String host, int validThreadNum){
 
         ProcessInstance processInstance = null;
         Command command = findOneCommand();
-
         if (command == null) {
             return null;
         }
         logger.info(String.format("find one command: id: %d, type: %s", command.getId(),command.getCommandType().toString()));
 
-        processInstance = constructProcessInstance(command, host);
-
-        //cannot construct process instance, return null;
-        if(processInstance == null){
-            logger.error("scan command, command parameter is error: %s", command.toString());
-        }else{
-            // check thread number enough for this command, if not, change state to waiting thread.
-            int commandThreadCount = this.workProcessThreadNumCount(command.getProcessDefinitionId());
-            if(vaildThreadNum < commandThreadCount){
-                logger.info("there is not enough thread for this command: {}",command.toString() );
-                processInstance.setState(ExecutionStatus.WAITTING_THREAD);
-                if(command.getCommandType() != CommandType.RECOVER_WAITTING_THREAD){
-                    processInstance.addHistoryCmd(command.getCommandType());
-                }
-                saveProcessInstance(processInstance);
-                this.setSubProcessParam(processInstance);
-                createRecoveryWaitingThreadCommand(command, processInstance);
+        try{
+            processInstance = constructProcessInstance(command, host);
+            //cannot construct process instance, return null;
+            if(processInstance == null){
+                logger.error("scan command, command parameter is error: %s", command.toString());
+                delCommandByid(command.getId());
                 return null;
+            }else if(!checkThreadNum(command, validThreadNum)){
+                    logger.info("there is not enough thread for this command: {}",command.toString() );
+                    return setWaitingThreadProcess(command, processInstance);
             }else{
-                processInstance.setCommandType(command.getCommandType());
-                processInstance.addHistoryCmd(command.getCommandType());
-                saveProcessInstance(processInstance);
-                this.setSubProcessParam(processInstance);
+                    processInstance.setCommandType(command.getCommandType());
+                    processInstance.addHistoryCmd(command.getCommandType());
+                    saveProcessInstance(processInstance);
+                    this.setSubProcessParam(processInstance);
+                    delCommandByid(command.getId());
+                    return processInstance;
             }
+        }catch (Exception e){
+            logger.error("scan command error ", e);
+            delCommandByid(command.getId());
         }
-        // delete command
-        delCommandByid(command.getId());
-        return processInstance;
+        return null;
+    }
+
+    /**
+     * set process waiting thread
+     * @param command
+     * @param processInstance
+     * @return
+     */
+    private ProcessInstance setWaitingThreadProcess(Command command, ProcessInstance processInstance) {
+        processInstance.setState(ExecutionStatus.WAITTING_THREAD);
+        if(command.getCommandType() != CommandType.RECOVER_WAITTING_THREAD){
+            processInstance.addHistoryCmd(command.getCommandType());
+        }
+        saveProcessInstance(processInstance);
+        this.setSubProcessParam(processInstance);
+        createRecoveryWaitingThreadCommand(command, processInstance);
+        return null;
+    }
+
+    private boolean checkThreadNum(Command command, int validThreadNum) {
+        int commandThreadCount = this.workProcessThreadNumCount(command.getProcessDefinitionId());
+        return validThreadNum >= commandThreadCount;
     }
 
     /**
@@ -669,7 +685,7 @@ public class ProcessDao extends AbstractBaseDao {
             paramMap.put(CMDPARAM_SUB_PROCESS, String.valueOf(processInstance.getId()));
             processInstance.setCommandParam(JSONUtils.toJson(paramMap));
             processInstance.setIsSubProcess(Flag.YES);
-            this.updateProcessInstance(processInstance);
+            this.saveProcessInstance(processInstance);
         }
         // copy parent instance user def params to sub process..
         String parentInstanceId = paramMap.get(CMDPARAM_SUB_PROCESS_PARENT_INSTANCE_ID);
@@ -677,7 +693,7 @@ public class ProcessDao extends AbstractBaseDao {
             ProcessInstance parentInstance = findProcessInstanceDetailById(Integer.parseInt(parentInstanceId));
             if(parentInstance != null){
                 processInstance.setGlobalParams(parentInstance.getGlobalParams());
-                this.updateProcessInstance(processInstance);
+                this.saveProcessInstance(processInstance);
             }else{
                 logger.error("sub process command params error, cannot find parent instance: {} ", cmdParam);
             }