Abstract: Time variable spectra of strong ground motion records are estimated based on the harmonic wavelet transform. The characteristic of time variable spectrum of the strong motion records on the sites of three classes are analyzed. It is shown that the records on hard soil site have shorter stationary duration in time domain, and have higher frequency center and wide frequency band in frequency domain. The average time variable spectra of strong motion records are simulated by uniform nonstationary stochastic model and time variable modified KanaiTajimi nonstationary stochastic model. The empirical formulas for the time variable spectra of strong motion records on three class soil sites are established by using nonlinear function fitting method. In order to demonstrate the precision of the simulated stochastic model, an error function is defined and the errors are calculated. The time variable spectrum factors corresponding to relevant seismic intensity and soil site class,which are compatible with the seismic code, are given. Finally, the artificial nonstationary ground motion is generated by solving the time variable linear differential equations.