Based on the analysis of the existing State of charge (SOC) Estimation Algorithms of batteries, this paper proposes a hybrid estimation algorithm according to the UKF method, the Ah method and the open circuit method in SOC estimation of battery pack which consists of 320 batteries. The “buckets effect” exists between series batteries and the differences between each cell were also been taken into consideration during estimating the SOC of power batteries. Finally, the hybrid estimation algorithm was applied to estimate SOC of battery pack under ECE driving cycles. The result shows that the error of the hybrid estimation algorithm is less than 5%.