The present invention provides a SOC estimation method applied to a battery system comprising a battery pack. The SOC estimation method comprises the steps of: determining an initial SOC value; determining whether the battery pack is in a working status; measuring the voltage and current of the battery pack if the battery pack is in the working status; calculating a current SOC value by using an ampere-hour method based on the initial SOC value and the measured voltage and current; determining dynamic characteristic parameters of the battery pack; and optimizing the current SOC value by using extended Kalman filter (EKF) method and based on the dynamic characteristic parameters of the battery pack.