The backlash of gear influences the pointing accuracy and tracking accuracy. Backlash could be eliminated by applying bias torque in multi-motor driving system. Dual bias driving system was taken as research objective. The mathematics model of dynamics was established, and many nonlinear factors which might affect the dynamic behavior were considered including gear backlash, stiffness of the gear train, random wind disturbing and cogging torque of motors. Minimum of bias torque was taken as the objective function. The driving gear and driven gear didn’t separate during work, which was taken as the bound for objective function. Moreover, the optimum bias torque under every driving torque at full range of speed was calculated. The changing rules of the optimum bias torque charged by each gear train parameter including inertia, stiffness and damping were analyzed and achieved.