To study the effects of gear impact, operating conditions and other factors on tooth root stress and gear contact stability, and clarify the correlation between these factors, the gear meshing impact force under variable operating conditions was calculated by using the energy conservation theorem, and the tooth root stress under different torques was calculated by using the modified tooth root stress calculation method. Taking a pair of gearbox gears as an example, the finite element method was used to establish the dynamic meshing model of gears under real working conditions,the dynamic contact force of gears was calculated, and it was compared with the theoretical value to verify its rationality. Subsequently, the effects of different working conditions on gear meshing impact force, tooth root stress, and contact stability were analyzed separately. Finally, the correlation between speed, torque, meshing impact force, and tooth root stress was comprehensively analyzed, elucidating the law of gear contact stability changing with working conditions, providing reference for selecting the optimal working conditions and optimizing contact stability of the gear system. The analysis results show that the change of torque causes the joint change of contact force, tooth root stress and meshing impact, and its influence on contact stability is greater than that of speed.