Abstract: Shaoguan is rich in hydropower resources. Most of them supply mountainous areas with power in the form of distributed small hydropower,and the grid structure in the power supply area is weak. When the main power supply line fails,the mode of splitting small hydropower units is often adopted,resulting in a waste of hydropower resources and reducing the power supply quality of residents in mountainous areas. In order to improve the operation efficiency of small hydropower stations and ensure the power supply reliability of residents in mountainous areas,a frequency modulation strategy of small hydropower microgrid considering parallel off grid mode switching is proposed in this paper. Fourier algorithm is used to analyze the signal flow,accurately obtain the unit frequency,realize and identify the off-grid state through frequency criterion,and adjust the working mode of hydropower unit in time. Discrete PID control algorithm is used to improve the stability of unit output frequency during off-grid operation. The application research on Ru-yuan 10 kV dong-xia-shan branch line verifies the effectiveness of the proposed strategy for the frequency modulation control of small hydropower microgrid,and has good application and popularization value for power grid in multi-hydropower mountainous areas.