According to the characteristic of the rotor system of long & thin shaft, ａdynamics model of rotor system under the mutational torque excitation is established. The simulation and the solution are carried out. Time domain and frequency domain waveforms of axis displacement are analysed. The result shows that the axis position is shifted under the mutational torque excitation, and the shift magnitude is determined by excitation size. The axis path is affected and it is un stable at the initial stage of motivation. The axis path gradually becomes stable with torque being stable. The test is set with the same simulation conditions. The correctness of the theory test is verified. The error between the measured curve and the theoretical curve shows that the transverse vibration response affects the cou pling stiffness .