Abstract: Considering the nonlinear oil-film force and oil lubricant viscosity, a dynamic model was set up for a rotor-bearing system with rub-impact faults. Its characteristics of rub-impact faults was analyzed by Runge-Kutta numeral simulation. Some ways to chaos was found, such as the motion of double periodic bifurcation and intermittent motion. Furthermore, the influence of changing the viscosity of oil in bearings on the dynamic behavior of a rotor-bearing system with rub-impact fault was investigated. The results show that when the angular speed of rotating rotor exceeds the double critical angular speed the rotor's responsivity is improved in the chaotic region with the decrease of oil viscosity. However, the reduction of oil viscosity affect its dynamic behavior much less if the rotor motion is in the region of sub-critical angular speed. The results provide a reference basis for the safe operation and the failure diagnosis of a rotor-bearing system with rub-impact faults.