The invention provides an off-grid and grid-connected smooth switching method of a virtual synchronous generator. The smooth switching method is characterized by comprising grid-connected/off-grid and off-grid/grid-connected smooth switching. As for off-grid/grid-connected switching, the switching strategy is added in the off-grid and grid-connected switching process without modifying the control strategy of the virtual synchronous generator, and the reference voltage of the virtual synchronous generator is changed into the command reference voltage from the grid voltage and the correction angular velocity of pre-synchronous control is enabled to be zero directly so as to realize grid-connected/off-grid smooth switching. Popularization of the virtual synchronous generator technology is further facilitated in the field of new energy power generation, and friendly accessing of distributed energy resources can be realized so as to enhance the stability of the electric power system. Besides, application of the control strategy and the theoretical analysis method related to the conventional synchronous generator to VSG is facilitated so as to play an important role in the active power distribution network and the microgrid in the future.