Abstract: By using the Hilbert-Huang transform(HHT),a newly developed time-frequency analysis method for nonlinear and nonstationary time series is proposed,the monitoring data is analyzed in time-frequency domain.The monitoring data is decomposed into several intrinsic mode function(IMF) components fluctuating at different frequencies and a residual representing the trend of the data through the empirical mode decomposition(EMD).IMF components exhibit amplitude modulation and frequency modulation.The residual can reflect the local feature of the trend.The Hilbert spectrums of the monitoring data of environment variables and effect variables are similar.The amplitude modulation and frequency modulation of monitoring data reflect the nonlinearity and nonstationarity of the complex system composed of dam,rock and environments.