Abstract: The structural seismic vulnerability analysis is crucial to evaluate the loss of earthquake disaster. Based on the incremental dynamic analysis (IDA) method and the theory of performance-based design, the structural seismic capacity probability function can be derived from the definitions and specified values for damage states in seismic codes, and seismic demand probability function can also be obtained by IDA. The vulnerability analysis method is put forward based on design performance, by which the seismic response of integral structure can be assessed effectively. The seismic vulnerability analysis of a steel frame structure is carried out with floor slab effect considered or not. Failure state assessment and seismic risk analysis are performed from the perspective of probability of damage. Finally conclusions are drawn that the existing of floor slab will lead to the reduction in structural ductility, seismic capacity, and the capability to resist structural collapse. The results provide bench mark reference for seismic design, reinforcement and damage evaluation after earthquake disaster.