The invention discloses a wind turbine generator torque arm displacement detecting device. The wind turbine generator torque arm displacement detecting device comprises a displacement sensor arrangedon a torque arm and an information collecting device connected with the displacement sensor, and the information collecting device is connected with a wind turbine generator master control system. Theinvention further discloses a wind turbine generator flexible support failure evaluation method. The wind turbine generator flexible support failure evaluation method includes: calculating wind turbine generator gear box torque T, and calculating force F borne by flexible support according to the torque T; using the torque arm displacement value S detected by the torque arm displacement detectingdevice to calculate the rigidity value K of the flexible support according to K=F/(S-S0); comparing the rigidity value K of the flexible support with a preset failure rigidity value, and judging thatthe flexible support fails if the rigidity value K is lower than the preset failure rigidity value. The invention also discloses a wind turbine generator gear box monitoring method. The real-time rigidity value of the flexible support is calculated according to the monitoring data of the displacement sensor mounted on the torque arm and the generator operation process, whether the flexible support fails or not can be monitored, and good operation of the gear box can be guaranteed.