Abstract: A simplified incremental dynamic analysis (IDA) method is presented in which the assumption of equivalent SDOF system is combined with the method of Pushover analysis. The damage index R is defined, and the corresponding relationships between R and the limits of structural response and aseismic three levels are proposed. The method for calculating seismic damage probability of structures is given according to the failure probability formula of structures, and the damage index expected value R_e of the structure under different earthquake intensity is obtained, which can be used to evaluate the earthquake damage grade of structures. On this basis, a simplified IDA based method is established for evaluating seismic damage of structures. In order to verify the proposed method, three structural models are designed according to different soil sites, and typical earthquake records are selected as the horizontal ground motion excitation. Through the implement of IDA, seismic vulnerability analysis and earthquake damage assessment on the structure models are performed. The analysis results show that the method of seismic damage assessment based on simplified IDA performs a small amount of calculation, and is easy to operate, but the obtained damage values are relatively large and the results of seismic assessment are partial to safety.