When using the Ah counting method to estimate state of charge of battery on electric vehicle, it is difficult to measure the initial SOC, and will bring accumulated errors. In order to solve the above problem, we design an algorithm which integrates the open circuit voltage method (OCV), Kalman filter (KF) and Ah counting method (Ah) in this paper. With the help of Matlab/Simulink platform, a test is designed to simulate the above algorithm, which shows the feasibility and accuracy of the algorithm were better than the Ah counting method alone.