Energy balance is the basic requirement in designing and evaluating the power system of nano-satellite, and it is often examined by numerical simulation on ground. The architecture and feature of the board nano-satellite power system was introduced first. Then the mathematical models of solar array, Li-ion battery and satellite load were developed respectively. At last, the energy balance was simulated by combining the satellite attitude dynamics. The simulation result can reflect the power change, battery capacity and state of energy balance in real time. The data of on-orbit experiment is consistent with the simulation results, which show that the models are correct and the simulation method is feasible.