Finally, I find the bug.See the debug picture as below
My project has no TransitionMap, so there is no chance to check the condition “CurrentMapName == DestinationMapName” to do “Connection->ClientWorldPackageName = NAME_None;”
So there are two solutions:
1. Add a Transition map to the project
2. modify the “FSeamlessTravelHandler::StartTravel” in World.cpp
@Sean L 
Is this a engine bug?