Due to the impact of natural factors, the output power of off-grid photovoltaic microgrid source is intermittent and random. This will lead to fluctuations of the bus voltage. To ensure the reliability and power quality of power supply, configuring a certain capacity of storage devices becomes necessary. This paper proposes and designs a PV program which the flywheel energy storage system is applied to as an auxiliary and control strategy that stabilizing the DC bus voltage in sections. Charge/discharge link of the drive motor for flywheel and its control method is analyzed and deduced based on theory of sliding mode control. A corresponding simulation module is constructed in Matlab/Simulink simulation environment. Controlling the bus voltage in two ways in the simulation, including the traditional PI control and sliding mode control, the results demonstrate the superiority of the sliding mode control method. The voltage quality of PV microgrid is effectively improved. This work is supported by National Natural Science Foundation of China (No. 51177047).