Internal charging effect and discharging effect in a dielectric material are one of the key factors threatening the spacecraft safety. Most of the spacecraft charging models could calculate only the voltage variation and electric field distribution in the spacecraft internal charging process, without estimating the consequence of discharging, such as the magnitude of voltage drop and discharging current. In this article, we present a combined model which could calculate both the charging effect and the discharging effect on polymer. The model is verified by the experimental results, and it shows that the residual voltage after discharging and the pulse width of discharging current grow with the increase of polymer resistivity. The magnitude of discharging current is determined by the breakdown electrical field.