Abstract: In this paper，a dynamic model of straight spur gear system with high contact ratio (HCR)，multiple clearances and time-varying meshing stiffness was established，the energy equations of the model were established based on the principle of work and power，the meshing stiffness of the gear system was calculated by means of finite element method，and the dynamic bifurcation and jump characteristics of the system were investigated by using the Runge-Kutta method． The results show that (1) there are abundant dynamic bifurcation characteristics in the HCR gear system influenced by multiple clearances; (2) with the increase of rotation speed，single-period，multi-period and chaotic motions appear in the system，and the motion state jumps from chaos to period motion in a crisis manner; (3) there is an obvious jump in the chaotic area; (4) the backlash greatly affects the system; for instance，the motion state is periodic when the backlash is less than 3. 85 × 10^－5 mand is chaotic in most areas when the backlash is larger than 3. 85 ×10^-5m; (5) the motion state of the gear system is in the crisis area of chaotic and periodic motions at low damping ratios and may get into single-period stable state via the channels of reversing bifurcation at high damping ratios; and (6) the driven clearance greatly affects the motion state of the system，while the driving clearance only has little influence on the motion state．

Key words: gear dynamics, high contact ratio, clearance, bifurcation, jump, stability analysis