Abstract: Determination of the elastic-plastic input energy for a SDOF system is a primary issue in the energy-based design approach. The paper selects 188 seismic records according to the site classification of Chinese seismic code, and then proposes three-segment elastic input energy spectra based on normalization methods. On this basis, the effect of strength reduction factor and damping ratio are analyzed on the elastic-plastic input energy spectra. The results indicate that the strength reduction factor and the damping ratio have significant impact on reducing energy spectrum peak value;the weakening action of damping ratio is impaired when strength reduction factor is bigger. According to the statistics of the influencing factors on elastic-plastic energy spectrum, the correction coefficient of the peak and long period segment is put forward through the 2-order fitting. Process to determine the elastic-plastic SDOF system energy spectra are suggested according to the study.