Virtual synchronous generator (VSG) technology is a kind of grid-forming control strategy which can provide voltage and frequency support. With the increasing proportion of new energy in power generation capacity, it has attracted extensive attention. First, its mathematical model is established. Then, to solve the problem of voltage non-synchronization, an additional pre-synchronization unit is proposed. On this basis, the control block diagram based on VSG is proposed. It can ensure the stable and safe operation in both grid-connected and island modes. Besides, it can realize the seamless switching between the above two modes. The simulation results show that the control strategy is effective and correct.