Aiming at the problem that the rotor mass imbalance has a strong impact on the safe and stable operation of the wind turbine generators, the impact of the fault on the motor speed and generator power is derived theoretically based on the description of the mechanism of the failure. Matlab/Simulink model which contains the wind turbine model, driven train model and the generator model under the fault state is established. Setting up different simulation examples from the quantitative point of view, signal feature and change curves of motor speed and generator power in both time domain and spectrum domain are analyzed accordingly. And the motor speed and generator power signal are obtained which have detected the 1st frequency and 2nd frequency of the rotor under the fault. With the imbalance increasing, the amplitude of 1st frequency and 2nd frequency is increased too. In the turbulent wind speed conditions, the motor speed and electromagnetic power spectrum in the low frequency band show continuous spikes. The method can be applied to the direct-driven wind turbine fault diagnosis system to identify the rotor mass imbalance and determine the size of the imbalance mass. This work is supported by National Science and Technology Support Program of China (No. 2015BAA06B02).