Accurate estimation of charging state of the power lithium battery is an important function in the battery management system of electric vehicle. In this paper, based on the second-order resistor-capacitance equivalent circuit model, an accurate charging state estimation of power lithium battery was investigated. State space expression was established firstly, and the parameters of equivalent circuit model were identified by the least square method. The relationship between open circuit voltage and residual charge was fitted by polynomial fitting method. By the usage of cubature Kalman filter, the state of charge of lithium battery was estimated at the same time. In the experiment, a digital signal processor-based charge and discharge platform was constructed. And the experimental results show that, the cubature Kalman filtering algorithm can achieve real-time online estimation, and the maximum error is less than 2%, which has high estimation accuracy.