Effective detection of unplanned islands in microgrids is one of the key problems to be solved urgently. This paper proposes an active islanding detection method based on frequency disturbance. After the inverter in the microgrid is successfully connected to the grid, the added frequency disturbance does not affect the power quality of the power distribution system. Once the distribution network is out of power, the frequency of terminal voltage of the Point of Common Coupling(PCC) fluctuates instantly and quickly exceeds the limit value, indicating that the islanding detection is successful. After successful detection, the frequency modulation module is used to quickly match the power of the inverter in the microgrid with the load power, and quickly realize off-grid switching. A microgrid model was built in MATLAB/Simulink, and the proposed method was simulated and verified through a variety of complex working conditions.