Abstract: The influence on the accelerating electromagnetic field caused by the accelerating apertures, cathode hole and supplementary aperture(the second acceleration type), is studied by using the MAFIA program. The calculation indicates that ignoring this influence is reasonable for simulating the electron motion. A C language program is compiled for the simulation, which follows the electron track of the whole motion and take the energy spread as criterion of the stable acceleration, so as to determine the range of parameters of microtron. A group of detailed result of S band microtron is also given.