Abstract: In the long-term operation of electron Linac, we encountered the continuousconduct problem of main thyratron in high power pulse modulator. It can causerandom fault. The reason is that synchronization trigger to provide trigger pulsemay produce random pulse, therefore a lot of anti-interference are used in synchro-nization trigger. A "Self-locking circuit" is specially designed. By means of thesemeasures, the problem of random fault in high power pulse modulator is solved,and stable operation of electron linac is assured.