VSC-HVDC (voltage source converter-based high voltage direct current) transmission technology has been successfully applied to large-scale new energy grid connection, grid interconnection and island power supply. However, in the process of switching between islanding and grid-connected control modes, there are still problems such as complex switching logic, and current surge caused by the position change of the grid-connected switch being not synchronized with the control logic switch. This article proposes a mode switching method for the Nanao VSC-HVDC Project to improve the success rate of control modes switching, adding a "transition state" based on droop control during the switch from island to grid connection. A new converter control structure is proposed for the "transition state". The current droop control is used as the inner loop, and the AC voltage amplitude and frequency are the outer loop. It has the external characteristics similar to the synchronous machine, stable operation under the two conditions. Using the "transition state" to decouple the AC switch position and the switching time of the control structure can improve the switching success rate, and virtual impedance is added to change the system pole configuration. A mathematical model is established to analyze the improvement of the system response characteristics by the additional poles of the virtual impedance. The proposed droop control strategy and control mode switching method are simulated and engineering verified to verify their effectiveness and correctness.