Abstract: The tool to study the nonstationarity of ground motion is time-frequency localization analysis,with wavelet theory as the prominent feature.A mathematic model of the relationship between wavelet transform coefficients and ground motion power spectral density function is builded based on harmonic wavelet.The time-frequency spectrum analysis of EL-centro seismic records is carried out,which verifies the model is reasonable.Then the power spectrum analysis of 455 acceleration ground motions recorded by the 2001 Yongsheng earthquake in Yunnan Province has been carried out,and the characteristic parameters of ground motions in this area have been obtained.Finally,based on the obtained a large number of power spectrum curve in periodic domains,a three-stage target power spectrum function has been explored.