This paper presents the electrostatic PIC-MCC model for simulating the breakdown of dielectric surfaces illuminated by high-power microwave，and the processes of multipactor and collisional ionization in the case of vacuum and different pressures are simulated by using the code developed by ourselves. The numerical results show that the multipactor effect is the main source of electrons，and the number of the electrons oscillates in the time domain at twice the rf frequency in the case of vacuum and low pressures. In the case of high pressure，the collisional ionization is the main source of electrons.