When the structure generates large local deformation under blast loading, the single degree of freedom (SDOF) method will have large errors or even mistakes to calculate the overall displacement of structure. In order to solve this problem, based on the SDOF theory, study on I-shaped steel beam, after analyzing the influence of local deformation of flange on the overall displacement of steel beam, the bending resistance function is improved based on the proposed simplified deformation model of steel beam section. An improved single degree of freedom (ISDOF) method for blast resistance analysis of steel beams is proposed. Finally, the dynamic displacement response and failure characteristics of steel beams under different blast loading are numerically simulated by the finite element software ABAQUS. The results are compared with those of the SDOF model and the ISDOF model, and the displacement and failure differences of steel beams under different calculation models are studied. The results show that compared with the finite element simulation results, the larger the local deformation of the steel beam flange is, the smaller the SDOF model calculation result is. The calculation results of ISDOF model are better, ISDOF model can be better applied to the simplified displacement calculation of blast resistance analysis of steel beams.