The invention relates to an off-grid and grid-connection control method and system for a virtual synchronous generator. A voltage error is calculated through a difference value between the grid voltage and the generator terminal voltage of the virtual synchronous generator, an electromagnetic equation and uncertain factor items; a sliding mode surface is designed according to the difference value between the grid voltage and the generator terminal voltage of the virtual synchronous generator and the voltage error; the difference value between the generator terminal voltage of the virtual synchronous generator and the grid voltage is controlled to be decreased according to the voltage error and the sliding mode surface through a fast terminal sliding mode control algorithm; when the voltage error is within a preset off-grid and grid-connection switching error range, off-grid and grid-connection switching is performed on the virtual synchronous generator. According to the off-grid and grid-connection control method and system for the virtual synchronous generator, due to the fact that when off-grid and grid-connection switching is performed on a generator terminal of the virtual synchronous generator, the generator terminal voltage of the virtual synchronous generator and the grid voltage are basically synchronous and a large impact current cannot be generated, the virtual synchronous generator can achieve seamless switching.