It is an essence reason that vibration and noises of gear system is rose by gear meshing dynamic excitation. The response analysis in inner dynamic excitation is important to the design and application of gear system. The time-variable stiffness curves of meshing teeth pairs are acquired by 3-D contact finite element method to speed increase gearbox , and the profile error curves is simulated by gear windage which is determined by gear precision rate. Stiffness excitation and error excitation can be obtained with it. Using the finite element analysis software (I-DEAS), the dynamic model of speed increase gearbox is established. Then the normal frequencies of whole gearbox are calculated, and the vibration responses of gearbox and transmission shafts are analysed. The results show that sympathetic vibration is not occurred in the use process of speed increase gearbox and the vibration swing is small. It is sufficient for systemic application demand.