Quantitative assessment of seismic damage by using damage models has been proved feasible and effective in controlling the earthquake-induced damage of structures. In this study, a modification is proposed for the original Park-Ang model by eliminating its non-normalization problem. The combination coefficient of the modified model is derived using the cyclic test results of flexure-dominant RC members from the database provided by the Pacific Earthquake Engineering Research Center and the author's own tests. An empirical formula is developed through multivariable nonlinear regression analysis to relate the combination coefficient with three design parameters. The comparison between the modified model and the original model indicates that the damage index of flexure-dominant RC members can be determined by the modified model with higher precision and smaller scatter. At last, the damage indexes at principle damage states are calculated by the modified model.