In this paper, the basic mechanism of spacecraft internal charging was introduced, and proton's effect on internal charging was mainly studied. Monte Carlo method is used to simulate the transporting process of proton in dielectric and the internal charging results in certain flux environment for the case of simple plane dielectric were calculated. Results indicate that the maximum electric field is correlated with the energy of the inject protons, and the electric field reached its maximum value under certain energy; that for the same number of injected particles and the same material parameters, the maximum electric field produced by proton is much higher than that produced by electron. Proton and electron data on 26th July, 2004, the time just before TC-2 satellite's attitude control system becomes invalid were chosen to calculate the internal maximum electric field produced by protons and electrons. The results reveal that the maximum electric field produced by protons is 2~4 magnitudes smaller than that produced by electrons, mainly due to proton's much smaller flux than that of electron.