Taking the symmetrical single disk rotor model as research object, the vibration of the rotor running at critical speed is studied and development of resonance is deduced. Two methods for identifying damping ratio of the rotor are proposed. In the first method,which uses the decay signals of free vibration, error function is constructed based on the envelope of the curve. Iteration method is used to minimize the error function, which can identify damping ratio more accurately then logarithmic decrement method. Amplitude-frequency curves of rotor, which is drawn according to the measured vibration data of transit process, is applied in the second method. Least square method is used to predict damping ratio, then, optimization method is utilized to determine the damping ratio. Experimental results show that both estimation methods can estimate damping ratio effectively and solve the practical problems.