High power microwave (HPM) has important applications in controlled thermonuclear fusion heating, microwave high-gradient accelerator, high-power radar, directed-energy weapon, super jammer, impact radar, etc. The window breakdown of HPM has been extensively studied, and some research progress in this respect has been made. However, the researches on the transition of window breakdown from multipactor discharge to rf plasma are still not enough in-depth. Especially, the influences of microwave frequency and microwave amplitude during breakdown need further studying. This paper focuses on the process of dielectric multipactor and background argon ionization during the discharge breakdown near the HPM dielectric window/vacuum interface. A one-dimensional-spatial-distribution-and-three-dimensional-velocity-distribution (1D3V) electrostatic model with using particle-in-cell simulation is adopted in present work. The model includes secondary electron emission, electrostatic field induced by the remaining positive charge on the dielectric window, the motion of charged particles under electrostatic and microwave field, and the collision process between electron and background gas, and the corresponding PIC/MCC code is also developed. We examine the effects of gas pressure, microwave frequency and microwave amplitude on discharge breakdown. It is found that there exists only electron multipactor process during the discharge breakdown on dielectric window in vacuum. At low pressures (10 mTorr, 500 mTorr) and slightly high pressure (10 Torr), electron multipactor and gas ionization are coexistent. However, at an extremely high pressure (760 Torr), the gas ionization dominates the breakdown process. At the same time, the position of plasma density peak moves away from the dielectric window as the gas pressure increases, which is the consequence of the competition between secondary electron multiplication on the dielectric window and gas ionization in the body region. It can be seen that the advantage of gas ionization gradually increases as the gas pressure increases. In addition, it is also observed that at 500 mTorr, the moment of gas ionization moves forward first and then backward with the increase of the microwave amplitudes or the microwave frequency, especially when the increment of frequency is numerically twice that of the amplitude, gas ionization occurs earliest. This phenomenon is explained by the secondary electron emission model. Meanwhile, the results show that the position of plasma density peak from gas ionization gradually approaches to the dielectric window as the microwave amplitude increases. However, with continually increasing the microwave frequency, the plasma density peak moves away from the dielectric window first and then approaches to the dielectric window.