In order to meet the analytical requirements of gearbox during variable speed process，a coupling dynamic modeling method for multi-stage gearbox is proposed based on the lumped parameter/finite element method，which is not only suitable for analysis of variable speed process，but also considers structural flexibility. To verify the effectiveness of the proposed method，the coupled dynamic model of helicopter main gearbox is established. The structural flexibility of the shaft and housing is considered in this model，and the time-varying mesh stiffness and mesh error are expressed as periodic functions of the gear rotation angles. The dynamic characteristics of multi-stage gearbox during the variable speed process are studied. The effects of constant wind，turbulent wind and gust on the helicopter main gearbox are revealed. The results show that the excitation and dynamic response of the system vary with the change of speed and torque. The excitation frequency and response fluctuation amplitude increase with the increase of rotational speed，and the excitation influence level and response mean value increase with the increase of torque. Compared with constant wind，the turbulent wind will aggravate the system. The vibration of the helicopter main gearbox is the largest under the action of gusts. The proposed modeling method is universal and can provide theoretical basis for dynamic design of gearbox during the variable speed process.