Abstract: Beijing Free Electron Laser Facility(BFEL)adopts a thermionic cathode microwave electron gun as its RF linac injector. For reletively long macro pulse operation, the back bombardment effect deteriorates the characteristics of the accelerated electron beam. So we developed a prediction based self adaptive feed forward control system to compensate for the beam loading. The system is operational and some experimental results have been obtained, which suggest that the system is effective to imp rove the beam quality, and that it's capable of dealing with complicated systems whose response is time-variable, nonlinear and of long delay.