To study the seismic vulnerability of steel portal frame structures based on the finite element model validation, the time-frequency conversion of time-history response of a measured portal frame structure under environmental excitation was carried out according to the power spectrum method, and the structural eigenvalues were extracted as the objective function for the finite element model validation. The finite element model was updated through multiple iterations with the complex method. The incremental dynamic analysis was conducted on the validated model, and the seismic vulnerability curves of the structure were then obtained. Based on β distribution, the seismic damage index expression was constructed and the optimization ratio curve of seismic damage index was drawn, and then the seismic performance of the structure with viscous dampers was analyzed. The results show that the finite element model validation theory can reduce the error caused by the uncertainty of structural parameters, thus making the performance state of the finite element model close to the real structure. Compared with the initial structure, the structure with viscous dampers has better seismic performance, and the seismic capacity of the structure can be improved by about 30%.