Starting from the basic principle of modal identification, and the expressions were derived for time frequency response of the excitation force and time response of resonant excitation, and the relationships among the timefrequency domain amplitudes were found. It was concerned that the associated free vibration in the resonant response is difficult to decay in a short time, and it was pointed out that the amplitude of the impulse frequency response spectrum at corresponding frequency is equal to the amplitude of forced vibration under resonant excitation. Based on establishing relationship of response equivalence, the formula of damping ratio was derived by a limited spectral information of impulse response spectrum. The accuracy of response prediction and damping ratio estimation was verified by ship girder model. In addition, the first eight modal damping ratios’ tests of 2 typical ship plate element models of different materials were also completed, and the rapidity and reliability of the method were verified by the numeral calculation binding experimental response frequency spectrum.