A standing wave accelerator system, with the procedures of soft start and AFC frequency stabilization, needs 0.5 to 5 seconds from the issue of the beam order by the accelerator to the state that the beam current of the accelerator is stable. Therefore, the delay time is long and unfixed which affects the fast response capability as the detector of the radiation source. A fast beam device of the standing wave linear accelerator (fast beam device for short) advances a thinking that the microwave system and the electronic beam emitting system can start work at different times, i.e. the microwave system starts work before the electronic beam emitting system and the accelerator electronic beam emitting system is started by the beam order (by electronic gun) after the AFC is put into operation and the operation is stable and the accelerator emits the X-ray beam. Experiment has proven that the time from the issue of the beam order to the state that the beam current of the accelerator is stable is less than 100ms.