In light of the hypotheses that the signal is a stationary random process,there are two main kinds of identification methods for structural modal parameters,namely time domain method and frequency domain method (Li et al.2002).As in the dynamic test of the real bridge structure,the requirement of stationarity is hard to meet.Furthermore,the measured data of bridge dynamic test often present nonlinear characteristics if there are some damages in the structure (Shan et al.2011).In recent years,the modal parameter identification of bridge structure based on the contemporary time-frequency signal analysis has received more and more attention.