Abstract: Based on the elastic-plastic time-history earthquake analysis of a single degree of freedom (SDOF) system, this paper investigated the seismic force modification factor R of the shear-slip (SSP) hysteretic model for different site types, natural vibration periods, ductility, damping ratios and post-yield stiffness ratios. Compared with the elastic perfectly-plastic (EPP) model, the effect of energy-dissipating capacity on the R-factor is studied. It is found that the most important factor determining R is the ductility, while the effect of energy-dissipating capacity is dependent on the structural period T. If T is less than the characteristic period TgR, it has an obvious beneficial effect, while T is greater than TgR, its effect is negligible. The damping and post yield stiffness have only a minor effect in the short period range. In the long period range, the damping has negligible effect on R spectra. The characteristic period TgR is used to normalize R spectra to maintain the peak values round TgR. The probability distribution of R factor corresponding to the normalized period and the ductility is analyzed. Simplified formulae are proposed for R spectra which are expressed as multiplication of ductility, damping and post yield stiffness factors.