For grid-connected wind farms, the classical frequency modulation control method has the problems of weak anti-interference ability and easy off-grid unit. A frequency modulation control algorithm for wind turbines based on sliding mode variable structure was proposed. Firstly, based on the working principle of permanent magnet synchronous wind turbine, a wind turbine model containing large power grid was established. Secondly, in order to solve the problem of weak anti-interference ability under traditional proportional integral differential (PID) control, the classical sliding mode variable structure controller was introduced. Then, in view of the shortcoming of the classical sliding mode variable structure controller which leaded to more chattering, a sliding mode variable structure primary frequency modulation control algorithm based on sigmoid function reaching rate was proposed. Finally, MATLAB/Simulink was used to model and simulate the primary frequency modulation of wind turbines. Compared with the classical control algorithm, the feasibility of the proposed algorithm was verified. Under the circumstance of wind speed fluctuation and power grid failure, the sliding mode variable structure primary frequency modulation control algorithm is more effective.