Fluctuation of the voltage is always the major barrier to the effective application of photovoltaic energy. In this paper, the flywheel energy storage system (FESS) is added to PV system. Considering the fluctuation of photovoltaic energy and limitation of flywheel capacity, flywheel may store energy from the photovoltaic source or AC system, or release energy to load in order to maintain the stable voltage on DC bus. Five different system operating modes and corresponding control strategies are presented in this paper. Corresponding to the different system operating modes, simulation has been done and the control strategies of FESS have been proved valid.