The method comprises: generating first, second, third and fourth pulse voltages; according to the first and second pulse voltage, generating a firs and a second electron beam respectively having a first beam loading and a second beam loading; according to the third and fourth pulse voltage, generating a first and a second microwaves respectively having a first power and second power; using the first and second microwaves to respectively accelerating the first and second beams in order to generate a first and a second accelerated beams; using the first and second accelerated beams to bomb the target in order to generating a first and a second X rays each having different energy.