WEBENCH® Tools/TPS54620: In WEBENCH,simulation the recommend designer,the startup ware form increases with the oscillatory behavior

Qiang,

That behavior is due to minimum on time constraints, at start up the required on time is less than the minimum, so the controller alternates between minimum on time (too much) and skipping pulses (too little) until the output voltage rises higher so that the required on time is above the minimum.  You cannot completely eliminate that behavior.  You can tune the SS time and Cout to minimize it somewhat.