The different potential between two low orbit spacecrafts is a serious safety problem for the spacecraft docking. The charging mechanism of the spacecraft in the ionosphere plasma is analyzed, and the equivalent circuit model in physics is educed. Based on that, the active potential control method for the International Space Station (ISS) is analyzed and two different methods that make sure of the spacecraft is immune from different potential are proposed. In the end, the solution for the spacecraft potential monitor including the Langmuir probe and the surface potential monitor during the docking is advanced.