With more wind power integrated into power grid， its impacts on the main system voltage are more significant. Grid companies require wind farm to provide supports to the main system voltage and reactive power control. Based on this， a reactive power compensation control method of wind farm is proposed considering the voltage and reactive power demand in the main system. The low voltage side of wind farm substation is chosen as controlled point. The reactive power control is divided into four sections， namely， normal control， abnormal control， urgent control and off-grid control. Accordingly， three control modes are given， including abnormal control mode， urgent control mode and fault control mode. In implementation， the average voltage in a specific period is used to classify the reactive power control sections. Further， the control mode selected according to the average voltage difference between two different periods and the communication between the substation and wind farm. An actual system is analyzed. The results show that the reactive power of wind farm can be quickly regulated to support the main system， maintaining the controlled voltage within limits in a measurement cycle.