The state of charge(SOC) is an important parameter of the power Li-ion battery. As there exists accumulation errors in the current integration method to estimate the SOC and some other methods are usually very complicated, a practical engineering method to calibrate the estimation of battery SOC is proposed. With the analysis of battery characteristics, some methods are established to calibrate the SOC initial value, the total capacity and the accumulated error by combining the current integration method. By establishing the relationship of the battery terminal voltage and SOC, the battery characteristics is analyzed in the different charging phase of the constant current and the constant voltage to obtain a high-precision estimation of the battery SOC in a discharging cycle. The experiment results show that an accurate estimation can be obtained and the estimation error is below 5%.