The research of dynamic mechanism of gearbox fault is one of the most important research contents of wind turbine fault diagnosis. Based on an improved lumped parameter model, dynamic models for planetary gear with different tooth breakage faults, including the sun tooth breakage fault, the ring tooth breakage fault and the planet tooth breakage fault, are built by modelling the faults with time varying mesh stiffness. The dynamics of a planetary gear at normal situation and under the above mentioned tooth break fault are simulated. The spectrum analysis and envelop analysis are conducted for the ring vertical vibration acceleration signal. The results show that, when external excitation in rotational direction alone is taken into consideration, the resonant peak in the vibration spectrum has a relationship with the modal energy distributions. In the tooth break states, the amplitude and intensity of the vibration signal increase greatly; modulation phenomenon occurs around the resonant peak; according to the envelope analysis, the modulated frequencies are the fault characteristic frequency of the corresponding tooth break fault and its harmonics.