By using a P3D3V PIC code programmed by the authors, the multipactor discharge effects on dielectric inner and outer surface under high-power microwave with TE10 mode in the BJ32 rectangular waveguide are numerically studied. The electron spatial distribution, distribution of electric field in the normal direction of the dielectric surface, and electron density spatial distribution are presented. Numerical results could be concluded as follows. For inner surface, the multipacting first occurs in the area with large electric-field of microwave; for the outer surface, multipacting first occurs in the area with small electric-field of microwave. The above phenomena could be explained as follows. Poynting direction of microwave is the same as the outer surface normal direction and opposite to the inner surface normal direction. So the drift in the area with large electric-field of microwave causes electrons easy to move back to inner surface, and so electrons are easy to leave from outer surface. Compared with 1D3V model, in P3D3V model, we have for inner surface multipactor discharge with long oscillator forming time, small secondary electron number, high average electron energy, low incident power of microwave, and low level deposited power; for outer surface, we have multipactor discharge with short oscillator forming time, small secondary electron number, low average electron energy, low incident power of microwave, and low level deposited power. The deposited power is about 1%–2% of incident microwave power both in 1D3V and P3D3V models; while the ratio between deposited power and incident power of microwave has nothing to do with microwave parameters and inner or outer surface.