A simple outgassing model for dielectric surface breakdown due to the high power microwave (HPM) irradiation was establised. By using the PIC (particle-in-cell)-MCC(Monto Carlo collisions) method, the numerical simulation program of the dielectrics surface breakdown was developed, and the simulation of breakdown for different outgassing speeds was performed. The temporal evolution of the electron number and delay time of the process of breakdown in the case of different outgassing speeds were obtained. The numerical results show that when the speed of outgassing is small, the breakdown does not occur for an HPM pulse of a given duration, because the formation of the gas layer on the dielectric surface is slow. When the speed of outgassing is greater than a certain value, the breakdown occurs and the breakdown delay time shortens with the gas desorption rate increasing in a certain range. Finally, the numerical simulation results were compared with the experimental results of the direct current dielectric surface breakdown, and the trends of their development agree very well.