Abstract: The nonlinear behaviors of rotor systems set high requirements to the designing of motor system:Such as to take nonlinear elements into account;to avoid the unstable parameter points or regions where will appear nonlinear phenomena.For this purpose,from studying a typical fractional model-May model and Jeffcott rotor system,the map of the fractional dimension of time-serial vs the parameter is obtained,which can identify critical bifurcated parameters of nonlinear dynamical system quite well.