The research of structural vulnerability analysis is mostly based on the Cornell theory. Using other methods to do seismic fragility analysis is relatively rare. In this paper, beta probability density function is used to continuously fit the probability distribution of damage levels for a frame structure under given seismic intensities. It gets the probability density distribution of the structure under different intensities. The results show that, based on beta probability density function, it can get comparatively accurate solution for structural seismic vulnerability analysis. This method is not only simple and fast, but also the damage state is expressed by continuous damage parameter. The research provides the reference for future earthquake damage prediction and the elaborate calculation about economic loss of earthquake insurance.