The models of conventional wind turbines with an asynchronous generator and power system are developed in DIgSILENT/Power Factory. Based on the model， the impacts of wind turbines with an asynchronous generator on the small-signal stability and damping characteristics of power system are analysed. An improved pitch control scheme is presented by introducing a frequency deviation signal into the control system. When low frequency oscillation happens， the wind turbine output is regulated to correlate with system oscillation frequencies. Both frequency domain analysis and time domain simulation results demonstrate that the improved pitch control methods significantly improve the damping characteristics of power system. Therefore the power oscillations are mitigated and the system dynamic stability is enhanced effectively in the system.