Abstract: In this paper, the oil film force of finite journal bearing isobtained using averaging natural value method. The nonlinear dynamicalbehaviors of a rigid rotor-bearing system were investigated with the help ofnumerical integral, Poinc\'are map and spectrum analysis. Some bifurcationmaps about rotating speed and a synthetically parameter which reflected theinfluence of many factors were drawn. The results show that complicateddynamical behaviors such as Hopf bifurcation, double-period and quasi-periodmovement are existed. The work above-mentioned lay the basis for averagingnatural value method to be applied to engineering structure.