A dynamics model of bending-tensional-axial-swing for two-stage helical gear transmission is developed, in which time-varying gear meshing stiffness, error and backlash are considered. The dynamic equation of the helical gear transmission is calculated and the dynamic response is gotten. The dynamic finite element model of gear case is established. The dynamic response of gearcase is calculated based on identification of the dynamic parameters of bearing combination. For the establishment of dynamic model of the whole gear system including two subsystems that are gear transmission and the box, the dynamic analysis of whole gear transmission system is realized.