Abstract: To avoid the limitations of the eigenvalues of matrix and Fourier series method for calculating stability and whirling trajectories of a rotor bearing system,the state variable equations analysis method in time domain is first proposed in this paper.Multiplying current state variable value by a matrix,the next time state variable value can be obtained by this method,which can select the large step size to satisfy a given accuracy.The typical whirling trajectories under position perturbation,impact excitation and synchronous unbalance excitations are simulated and a method is given to determine the stability threshold of a rotor bearing system.Simultaneously,the applicable condition of small perturbation theory is discussed.