The present invention provides a standing wave electron linear accelerator comprising a modulator and a magnetron for producing radio frequency microwaves; a plurality of accelerating tubes for accelerating electrons; a microwave transmission system for feeding the microwaves into the plurality of accelerating tubes; a plurality of electron guns for emitting electron beams into the plurality of accelerating tubes; a plurality of targets impinged by the electrons from a plurality of accelerating tubes to form continuous spectrums of X-rays; a plurality of shielding devices for shielding the continuous spectrums of X-rays generated by the targets; and a microwave distributor disposed adjacent to the end of the microwave transmission system, wherein the microwave distributor has a microwave inlet and a plurality of microwave outlets for distributing the microwaves in the microwave transmission system into the accelerating tubes.