Abstract: The time-domain differential equations are derived for spacecraft charging based on the equivalent circuit theory, and a related simulation program is developed with FORTRAN language. Under the worst-case geomagnetic sub-storm conditions, the surface charging levels of spacecraft in sunlight and without sunlight are analyzed. The electron beam ejecting outward from spacecraft as a way of controlling spacecraft surface potential is also analyzed. The calculation result shows that this approach by ejecting electrons from the hollow cathode plasma can effectively keep the spacecraft surface potential under control.