Seismic damage was the main reason for damage and collapse of structure. One of the development directions of performance-based astigmatic design was to quantize seismic damage and evaluated seismic resistance by damage analysis. This paper expounds the Park-Ang criterion, which is widely used for damage analysis, and its insufficient is pointed out. The modified Park-Ang two-parameter damage evaluation model is used in this paper, two kind of methods are adopted to define damage index. By analysis the damage conditions of typical rail transit bridges under various strength level in Beijing area, it is proved the result of two analysis methods have a good consistency. This paper offers a good reference for aseismatic performance evaluation and design of rail transit bridges.