Abstract: The work presented here investigates the nonlinear vibration characteristics of reinforced concrete beams.It is verified that free small-amplitude bending vibration of reinforced concrete beams contains weak square nonlinear terms by theory deduction and experiment.The instantaneous frequencies decay with amplitudes.The wavelet transform is used as a time-frequency representation for system identification.A series of impact-excited vibration tests on a reinforced concrete beam under different damage degree had been conducted.The impulse response of the concrete beam is transformed by Morlet wavelet and the instantaneous characteristics are obtained by the ridges of the wavelet coefficients.The nonlinearities are detected by examining the changes of the frequencies during vibration.The inherence frequency,damping ratios and nonlinear parameter of the system are identified by compound wavelet transform.The tests also demonstrate that the inherence frequency decreases while the nonlinear parameter increases with damage.