The conventional beam/shell elements cannot consider complex geometric features of components, leading to low calculation accuracy, so as the finite element calculation has a large amount of computation and its system-scaled modeling and dynamic analysis are difficult. For the abovementioned problems, a variable speed modeling method for the planetary gear train is proposed. Taking the planetary gear train in the low-speed stage of a 5MW wind turbine gearbox as the research object, the coupling relationship between the ring gear and its elastic support is established by using finite element condensation theory based on the ring gear structure and its boundary characteristics. Then, the rotation angle of the driving gear is associated with the meshing states of gear teeth in the planetary gear train by introducing the characteristic variables for describing the variable speed process of meshing gear pairs and virtual vibration line displacement of ring gear. Finally, considering the flexibilities of the carrier and sun gear shaft, a dynamic model of the planetary gear train that is suitable for describing the variable speed process is established by coupling each component with the compatibility of interface displacements. The result shows that when the loaded five planet gears reach equilibrium, the transient deformations of all ring gear tooth nodes are presented as the shape of a pentagram and rotate with the carrier. The dynamic tooth load of the planetary gears increases first and then decreases, which is similar to the meshing stiffness of a single tooth pair. Under the steady condition, the vibration displacement of ring gear tooth node shows the superposition characteristics of large low-frequency fluctuation and high-frequency vibration. When the input torque is suddenly changed, the dynamic load balance between multiple planets will be destroyed, and the flexible ring gear can absorb part of the component’s vibration caused by impact load to a certain extent, and improve the load sharing performance.