The battery energy storage system（BESS） is a promising solution to maintain the stability of the islanded power system with distributed generators（DG）， such as wind power and photovoltaic power. The principle of the stability operation and the energy balance for islanded power systems is analyzed. The energy balance and hierarchical control strategy composing of system control and local control are proposed for the islanded power systems with the doubly-fed induction generator（DFIG） and the parallel-connected battery energy storage system（P-BESS）. Both the operation state of the system and the operational principle of the P-BESS are introduced， and their control strategies are presented. Simulation and experimental results demonstrate that the voltage and the frequency of the system can be kept at an expected level and maintain its efficient and stable operation under different dynamic conditions， such as varying wind speeds and loads.