Abstract: Automatic frequency control (AFC) system is one of key systems of electron linear accelerator. However, the strong frequency mutation of the magnetron or the effects of temperature is likely to cause the magnetron frequency offset, which will lead to the failure of the AFC system and a sharp decline of accelerator dose rate. In order to overcome the existing problem of the current AFC system, an AFC system with the function of automatic frequency search was designed according to the feedback signal of magnetron tuning location and dose rate. The experimental results show that the system can effectively prevent the frequency offset and maintain the maximum output of accelerator dose rate.