The coupling modulation phenomenon between different gear trains of wind turbine gearbox interferes with the actual fault diagnosis. Therefore, a phenomenological model considering interstage coupling modulation is proposed for a two-stage planetary onestage parallel gearbox. Firstly, based on the amplitude and frequency demodulation analysis of vibration signal of wind power gearbox, the characteristics of series modulation between stages under multistage transmission are defined, and the series modulation model between amplitude-frequency stages is proposed. By constructing multistage amplitude-modulation signal and frequency-modulation signal, the series modulation characteristics are simulated in both the spectrum and demodulation spectrum, and the sideband energy index of coupling modulation characteristics is proposed for model evaluation. A scaled gearbox test bench with the same structure as the field gearbox is designed, and experiments and field analysis are carried out under both normal and fault conditions to verify the validity of the series modulation model between stages. The results show that the frequency coupling modulation phenomenon is particularly obvious in the four states, and the sideband energies of the coupling modulation characteristics are 1. 02, 1. 04, 1. 18 and 1. 25, respectively. The coupling modulation between gearbox stages reflected in this model is the gearbox itself, and will not change with the gearbox state. This model provides a reference for improving the fault diagnosis accuracy of wind power gearbox.