A new twenty-freedom non-linear dynamic model for two-span dissymmetric elastic rotor system was established by considering the crack fault, rub-impact fault and oil film force. The complex motion characteristics of this system were numerically studied and the effect of different parameters on the system is investigated. Moreover, the both cases with and without crack fault were discussed in detail. The results indicate that the system has obvious single-period, period-doubling bifurcation and chaos phenomena. There are marked influences of the crack faults on the nonlinear characteristics of the system, especially under some the rotating speed. The case considering crack faults has different chaos region from the case without crack fault. The chaos phenomenon appears at lower rotating speed, and when the chaos comes again, the corresponding rotating speed extends backwards. Synchronously, the influence of crack fault increases along with the depth of crack. The results may provide theoretical references for rotor fault diagnosis.