With the increase of wind turbine installation， the behavior of wind power system during grid disturbances becomes very important. Grid codes require that wind turbines stay connected to the grid during voltage dips for a certain period. A control method is proposed to keep direct-driven wind power system with back-to-back full power converters using permanent magnet generator connected to the grid during voltage sags. When voltage sags happen， the grid-side converter operates in the STATCOM mode and export reactive power to maintain the wind turbine terminal voltage. Simulation and experimental results illustrate that the method can effectively improve the direct-driven wind power system low voltage ride through capability.