The secondary electron multipactor on the inner surface of the output window is one of the main factors limiting the power capacity of high power microwave. Therefore, it is of great significance to carry out relevant research. In this work, the process of secondary electron multipactor and the resulting loss of power are numerically simulated by using the electromagnetic particle model with one-dimensional spatial distribution and three-dimensional velocity distribution at the microwave frequency of 110 GHz. The influences of microwave electric field at the surface and dielectric material type on the power loss are studied. The simulation results show that the electron number density is higher than the critical cut-off number density after the secondary electron multipactor has reached the steady state, but the microwave electric field does not show obvious change. This is because the electrons in a very high electrostatic field are mainly concentrated in the domain of several micrometers near the dielectric surface, which is far less than the corresponding skin depth. The electron number density in the multipactor steady state increases with the microwave electric field increasing, but the ratio of the power loss to the microwave power at the surface increases slowly. After the multipactor reaches the steady state, the number density of electrons near the sapphire surface is the highest, followed by the number density near the crystal quartz surface, and the number density near the fused quartz surface is the lowest, so the corresponding power loss decreases successively. In order to verify the accuracy of the model, the simulated value of the multipactor threshold is compared with the experimental data, and the difference between them is discussed.