Dealing with stability of torsional mode of grid - connected wind turbine generator including series compensation device, shafting torsional mode of the different mass are analyzed with the small disturbance method , which is based on establishing the three-mass equivalent model for shaft of wind turbine generator . The influence of stability of the wind generator Shafting torsional mode between different mass and series compensation degree level are based on matrix pencil method, and then analysis results show that shaft system may occur transient torque amplification in a fault condition. Wind turbine shaft system and generator electromagnetic transient model and other part of the grid model of an actual wind farm are built in the PSCAD/EMTDC. The simulation results show that transient torque amplification phenomenon occured in some grid fault, and verify the series compensation can affect the stability of torsional vibration mode.