Abstract: Five welded stainless steel I-section beams were subjected to three-point bending to explore the non-linear deflection behavior of stainless steel beams. Finite element (FE) models were developed by considering the material nonlinearity, initial geometric imperfections and welding residual stresses. The models were validated against the experimental results. Based on the validated numerical models, both the calculation method in the Chinese technical specification for stainless steel structures (CECS 410) and the proposal of Real-Mirambell were evaluated. A simplified calculation formula for the yielding moment M_0.2 of stainless steel cross-sections was proposed based on the plane section assumption and the stress–strain relationship of stainless steels. The decrease in the initial flexural stiffness resulted from the welding residual stresses was considered by introducing reduction coefficients for different stainless steel alloys. Revised formulas for the CECS 410 method and the Real-Mirambell proposal were presented, which provided accurate and reasonable deflection predictions for welded stainless steel I-section beams.