In this paper, the basic algorithm of S-transform is presented, and the time varying amplitude spectrum and the time varying energy spectrum reflecting the time-frequency characteristics of the seismic records are defined. The time-frequency characteristics of Northridge wave, El-centro wave, Kobe wave and an artificial wave are analyzed through numerical example. Earthquake records can be adjusted with time-frequency filtering based on S-transform, and a new procedure is presented to generate an accelerograms whose power spectrum or response spectrum is compatible with a target spectrum by using actual earthquake records as parents.