Specimens of cold-formed steel quadruple limbs built-up box section member were tested to investigate its load capacity under flexural loading, and a finite element model involving geometric large deformation, materials and contact nonlinearity was established for numerical analysis. The model was validated by comparing the failure mode, ultimate bearing capacity and load-deflection curves with the test result. Based on this proved model, a series of parametric analysis was carried out to identify factors affecting flexural strength, such as spacing of screws, the height and width of section, and the quantity of initial imperfections. A practical design method named reduction coefficient method based on effective width method was proposed and compared with current AISI and Chinese cold-formed steel codes. It shows that current AISI and Chinese codes lead to a conservative evaluation. The reduction coefficient method proposed in this paper shows good agreement with the finite element result, and is relatively simple and convenient for design.