Abstract: The commutation phenomena produced by cathode outgassing ionization is a possible factor limiting MILOs work performance, and also is the main obstacle to the MILOs repeat frequency. In this paper, the physical modeling technology for cathode outgassing ionization phenomena of the high-power microwave device MILO and the particle simulation techniques to achieve the desired three-dimensional self-consistent calculation are analyzed by three dimensional PIC code. According to the simulation and computing results of different relative outgassing rate, when the outgassing rate exceeds a certain threshold, the plasma caused by the ionization will make the microwave output power rapidly.