The dynamic modeling of a flexible rotor system supported on misaligned journal bearing was discussed and its nonlinear dynamics was studied. Under some assumptions such as short journal bearing, small bearing misalignment and tiny unbalance, first, a dynamic model with ten degrees of freedom was established after considering the flexibility of shaft and misalignment of bearing and unbalance of disk. And then the nonlinear dynamic characteristics of the rotorbearing system were analyzed by numerical method. The result shows that at low speed the steadystate responses of the rotor are synchronous with the rotating speed, and as the speed increases there exists the vibration components of integer multiple frequencies in displacement responses, at higher speed the displacement response becomes synchronous with the speed again.