Abstract: Monte-Carlo collision (MMC) method was introduced for description of the gas collision ionization course. The MCC gas ionization module based on the method was developed for 3D fully electromagnetic and PIC code NEPTUNE. The magnetically insulated transmission line oscillator(MILO) model with He gas was simulated by the developed NEPTUNE code. The simulated results indicate that, in the condition of lower He gas density, He+ ions produced by ionization collision can not move freely because of their weight, and form channels which can neutralize space-charge field perfectly and then improve the bunching and beam-wave interaction efficiency obviously, so the output power of microwave increases, and the oscillation forming time shortens markedly. When the density of He gas increases to a higher level, the numbers of electrons and He+ ions increase quickly because of stronger ionization collision, the energy of electrons is separated due to collision courses, which goes against the bunching and beam-wave interaction improvement, and then the output power of microwave is decreased or cut off. The shortening of oscillation forming time is induced by the initial ionization stage with lower electron and ion densities. With the increasing gas density, the output power pulse will be shortened, and the diode of MILO will be shut down. In addition, the MILO models with air, vapor and carbon dioxide gas were simulated, respectively. The simulated results indicate that, in these conditions, the shortening of output microwave pulse is more severe compared with that for the MILO filled with He gas at the same pressure.