Abstract: There is an ununiformity with the electron beam energy spread at Beijing Free Electron Laser Facilitis. It is observed that the electron energy in front of the current pulse rose much higher than the average,that this portion of electron bunchs lost in the bending segment and could not devote its energy to laser gain.This disfigurement is considered to be caused by the difference of building time of microwave field inside the RF gun and the accelerator.One cavity with low quality factor to shorten this difference was introduced in this paper,and an upgrade design of new thermionic cathode made of this kind of cavity is expected to improve laser gain.