Abstract: It takes single-degree-of-freedom systems as objectives. A set of 662 mainshock-aftershock sequences is selected as the earthquake inputs. The damage index proposed by Park and Ang is adopted as the damage indicator. Then the mainshock-damages, aftershock-induced damage increments and accumulated damage under the action of mainshock-aftershock sequences are examined at entire structural periods. An incremental damage ratio (i.e., a ratio between aftershock-induced damage increment and mainshock damage) is proposed. The correlation between the incremental damage ratio and the mainshock-aftershock intensity ratio (i.e., a ratio between aftershock intensity measure and mainshock intensity measure) is investigated. A prediction model for the incremental damage ratio is developed as a function of the mainshock-aftershock intensity ratio and of the structural period. The accuracy of the prediction model and the influence of different SDOF systems are discussed. The results show that the incremental damage ratios have a good linear interdependence with the mainshock-aftershock intensity ratio in the logarithm space. The prediction model for the incremental damage ratio shows sufficient accuracy. Moreover, the hysteretic models of SDOF systems have a significant effect on the accuracy of the incremental damage ratio prediction model.