The VSG mathematical model is analyzed,in view of the grid-connected converter with VSG characteristics, this paper is focused on the analysis of the pre-synchronization based on the step-by-step idea and active island method based on the disturbance of grid voltage. According to difference value between the system voltage amplitude and phase on both sides, voltage amplitude and phase of the converter is adjusted by a certain step in order to close to the grid side. When differences are in a small scope, grid voltage amplitude and phase are used as the reference value the converter, thus converter switching from off-grid mode to grid-connected mode with zero impact interconnection. According to active islanding identification of converter with VSG characteristics, the grid voltage amplitude is perturbed by small step. The new grid voltage amplitude is given as reference for the voltage of converter. If the grid is connected, the disturbance has no effect. If the grid is disconnected, voltage will continue to decrease, island is identified eventually. Through MATLAB simulation analysis, the feasibility and effectiveness of the proposed method is verified.