In order to solve the problem of low accuracy and high cost of state of charge(SOC) estimation for the Vanadium redox battery, an estimation method is proposed based on the combination of the recursive least squares(RLS) algorithm and extended Kalman filtering(EKF) algorithm. The RLS algorithm is used to estimate the battery model parameters, and the EKF algorithm is used to estimated the battery SOC. The battery SOC is estimated accurately by the combination of the RLS and EKF when the battery parameters are changed. Taking the 5kW/30kWh Vanadium redox battery as the object, the SOC estimation of the Vanadium redox battery is realized by using the proposed algorithm. The results show that the algorithm can estimate the SOC of the Vanadium redox battery accurately and save the cost of the SOC measure when an additional single testing cell is added.