Abstract: Multipactor is one of the bottlenecks for spaceborne high-power microwave components. In this work, based on 3D particle-in-cell simulation, we investigate the fluctuation of number of electrons, the instantaneous secondary electron yield, the normalized reflection wave voltage and the gap voltage between the dielectric surface and metallic plate in the evolution of multipactors with parallel-plate dielectric-loaded waveguide for three cases: just considering external microwave field (case 1), considering both external microwave field and space charge (case 2), and considering external microwave field, space charge and surface charge on the dielectric (case 3). The electron distribution and charge density on the surface of dielectric versus time for case 3 are given. Simulation results reveal the roles of space charge and surface charge on the dielectric: space charge results in the saturation state of the multipactor, while surface charge on the dielectric results in an unsustainability of the saturation, extinguishing the multipactor. The existence of surface charge on the dielectric is responsible for the special circumstances including the different decreasing rate of instantaneous secondary electron yield of dielectric and metal plate, the eye-like shape of envelope of the normalized reflective wave voltage amplitude, the gap-voltage DC-like bias, and the unsymmetrical distribution of the energy of electrons.