According to the structure characteristic of the magnetically insulated transmission line oscillator (MILO), a one-sided multipactor discharge model is present. Through the calculations of the susceptibility curves and phase focusing effect, the physical conditions of the multipactor discharge occurring in the MILO are analyzed. The results show that one-sided multipactor modes have a wider phase focusing range than two-sided modes, and there is many chances for electronic grazing incidence on the metal surface for MILO. So the multipactor discharge is more serious for MILO than O-type devices.