The power conversion system (PCS) based on droop control is voltage control in parallel. The island and grid-connected operation of the PCS are controlled by turning off and on the grid-side static transfer switch (STS). When the grid fails suddenly, the STS breaking command is issued. The PCS runs in island through the breaking process, but a longer time voltage shock, power and frequency oscillation will occur after breaking process is over. In order to solve above problems and achieve smooth transition of parallel PCS from grid-connected to island mode, a new type feedback link of voltage and current controller is designed in this paper. It combines the differential and integral controllers with droop controller to achieve smooth switching from grid-connected to island mode. In addition, the stability of the improved system is analyzed through root locus. The simulation results verify that the proposed control strategy can achieve a smooth transition from grid-connection to island mode.