To address the vibration coupling modulation problem caused by the multi-stage transmission of the wind turbine gearbox, a new ring gear fault model considering the inter-stage amplitude modulation and frequency modulation is proposed. It is applied to the monitor ring gear by the parameter identification technology. The fault characteristic frequency of a certain stage of ring gear will modulate the meshing frequency of different gear trains in a way of amplitude modulation and frequency modulation, which shows the phenomenon of coupling modulation. In the view of this special modulation law, this article establishes the coupling modulation model of vibration signal under two-stage ring gear fault. On this basis, the parameter identification technology based on local mean decomposition and Levinberg-Marquart algorithm is proposed to determine the amplitude modulation coefficient of the fault model. Then, the condition indicator can be easily constructed to achieve the purpose of ring gear fault monitoring. The field data are tutilized to evaluate the content of this paper. Results show that the new model describes the vibration signal more comprehensively than the traditional model. The index constructed by parameter identification technology can locate the fault gear.