The commutation phenomena produced by cathode outgassing ionization is a possible factor to limit work performance of magnetically insulated transmission line oscillator (MILO), and also is the main obstacle to limit its repeat frequency. In this paper, physical modeling technology of MILO cathode outgassing ionization phenomena and three-dimensional particle simulation are presented. The relation between remaining gas of previous outgassing and pulse duration is analyzed. When the number of positive ions is greater than that of electrons the plasma caused by the ionization will rapidly reduce microwave output power.