Abstract: The rapid development of photovoltaic (PV) and electric vehicle (EV) has caused power quality problems such as high voltage at the end of distribution network, and flexible multi-state switch (FMSS) technology based on regional flexible interconnection has become one of the solutions. Aiming at the fluctuation and impact of switching between PQ mode and VF mode of flexible multi-state switch, the study proposes to add inertia link and and improve phase pre-synchronization link after the state-following controller, which can realize the smooth switching of FMSS parallel and off-grid operating modes. A control strategy with the introduction of weight adjustment is proposed to realize the smooth switching of active control and DC bus voltage control under the stable operation of the system, in respond to potential operation collapse due to feeder failure at the DC bus control terminal of FMSS. A three-port FMSS simulation model was built in MATLAB/Simulink environment to simulate and verify the proposed control strategy, and the simulation results demonstrate the seamless switching function of three-port FMSS under any feeder fault. Finally, a physical experiment platform was built to further validate the effectiveness of the proposed smooth switching strategy.