To investigate strength reduction factor R and dynamic magnification factor β,dynamic response of single degree of freedom(SDOF) elastic-perfectly plastic(EPP) hysteretic system are analysed.The influence of site types,natural vibration periods,displacement ductilities,damping ratios and P-Δ effect coefficients are taken into account.The characteristic periods Tga and TgR are used to normalize the R spectra and β spectra.Results indicate that ductility,damping ratio and P-Δ effect coefficient all have obvious effect on R and β.β produces more ideal effect than R on elastic-plastic seismic force calculation.