This paper presents a novel approach for estimating the seismic performance of earthquake-damaged structures by damage index. The novel approach is based on the Park-Ang damage index formula method, the incremental dynamic analysis method and the method proposed by Ou Jinping and Wu Bo (1995) for estimating the restoring force of earthquake-damaged structure. A real bridge pier is taken as an example for numerical modelling, and the pushover method is used to extract the relevant parameters of the pier for the subsequent calculation of the relevant index. In the first step, the seismic records are selected and amplitude-modulated to be input into the numerical model to create a large number of damage states of the structure after shock. In the second step, the force-displacement response from the previous step is collected and the damage index is calculated by the Park-Ang damage index formula method. In addition, the skeleton curve of the restoring force in different damage states of the bridge abutment after shock was calculated using the oujinping method. In the third step, the fitting relations equation between the damage index and the relevant parameters of the damage skeleton curve was obtained by regression analysis. Through the above steps, we can model the mechanical model of bridge piers under different damage indices. Finally, the bridge piers under different damage states are analyzed by incremental dynamic analysis (IDA(Mahin and Bertero (1978))) and the relationships between seismic performance with damage indices were obtained. The results of the relationship between damage indices and seismic performance obtained by the above methods will provide effective suggestions for assessing the remaining seismic safety capacity of the structure and structural health monitoring.