Abstract: In view of strict requirement on smooth mode switching,anti-interference ability and demand response of system during grid-connected and off-grid operation of microgrid system,a dualloop control strategy of dual mode inverters under synchronous rotating reference frame of grid fundamental frequency was proposed,and a mathematic model of inverter grid-connected and off-grid mode in photovoltaic microgrid systems was built. When the systems are connected to power grid,voltage vector tracking current control strategy based on PI controller is adopted; in off-grid operation,an SVPWM-based dual-loop control strategy with capacitor voltage outer-loop and inductive current inner-loop was adopted in off-grid operation. To reduce grid impact of the mode switching,a smooth grid-connection and off-grid switching method was formulated. The simulation results show that the proposed control strategy can ensure operation stability of the system,reduce voltage and current impact and enable smooth mode switching.