Based on the virtual synchronous generator (VSG) technology, the off/on grid smooth switching control strategy of microgrid inverter is studied. Firstly, the mathematical model of VSG is established, and the control parts of VSG including power frequency regulator, excitation regulator and electromagnetic equation are designed. Secondly, the traditional off/on grid switching principle is analyzed. Due to the existence of phase-locked loop (PLL), the pre-synchronization speed and accuracy are greatly affected. Aiming at this problem, an improved pre-synchronization control method is proposed, which can replace the PLL section with only two PI regulators. Thus the number of control parameters is reduced, the switching instantaneous impulse current is small, and the active and reactive power impact is suppressed, resulting in the off/on grid smooth switching of microgrid inverter. Finally, the simulation results of MATLAB/Simulink verify the correctness and superiority of the proposed control strategy.