To solve the voltage stability and control issues caused by large-scale wind power integration into grid， a multi-mode and multi-objective reactive power control strategy is proposed for wind farms with variable speed and constant frequency wind turbines. Based on this， a reactive power control system for wind farm is developed. The design program of the system structure， main function modules and interfaces are introduced. A test is carried out at Shanghai Fengxian Haiwan wind farm. The result shows that the strategy is correct and the wind farm will provide reactive power support to the point of interconnection by regulating reactive output helping to maintain voltage stability in the grid.