Abstract: Considering the time varying stiffness and nonlinear oil film force, a dynamic model was established for the double disc rotor-bearing system with crack faults, and subsequently numerical integration method was used to solve the model, so as to study the effects of rotating speed, crack depth and unbalance force on the response, instability and bifurcation of the system by using bifurcation diagram, axis orbits, Poincaré maps and cascade spectrum. Results show that when the rotating speed changes, rich nonlinear dynamic behaviors would occur to the system, such as multiple periodic, quasi-periodic and chaotic motions. Under deep crack conditions, double period bifurcation and multi period motion would appear at a lower speed, when the system nonlinearity and instability tend to be enhanced. The increase of unbalance force simplifies the dynamic behavior of the system, makes the appearance of system instability lag behind, but does not affect the oil-whip.