Abstract: In this paper, the failure condition of steel is proposed based on the concept of ultimate strain point failure, and the failure function and damage surface are given. Taking a simply supported steel beam as an example, this paper attempted to apply the ultimate strain method of the numerical limit analysis to steel structures by using FLAC^3D software. In the method, the numerical limit analysis was adopted to solve the ultimate strain of materials and regard it as the failure criterion for material points. For both tension and compression, the steel is regarded as an ideal elastic-plastic material. The steel shall yield when it reaches the elastic ultimate strain under the yield load, and shall break under the elastic-plastic ultimate strain. First of all, a direct pulling test was conducted on direct-pulling steel test pieces whose material is the same with the steel beam to obtain the mechanical parameters of the steel material. Furthermore, the numerical limit analysis method was used to determine the ultimate strain of the steel material. Afterwards, a pure bending test was carried out on the simply supported steel beam until it is destroyed. FLAC^3D was utilized to simulate the test process. The numerical calculation results were compared with the test results. The results showed satisfying consistence in the failure mode and ultimate load-bearing capacity of the steel beam. It proved that the ultimate strain method can be applied to steel structures and further studies on complicated structures can be conducted. The failure standard of the steel beam based on the ultimate strain is compared with the current specification failure standard.