As the thermal power units often require a long time to response to load disturbance and have low unit ramp rate, a method of battery energy storage system assisted automatic generation control based on fuzzy control strategy is proposed. This fuzzy controller uses Area Control Error (ACE) and its rate of change as inputs and the reference power variation as output. BESS output power is adjusted according to the system's operating status, assisting to improve the dynamic performance of the grid frequency regulation. Simulation results based on Matlab/Simulink platform show that BESS responds quickly to load disturbance, thus reducing the system frequency deviation and tie-line power deviation, lowering the unit ramp rate requirements, and improving the stability power system.