Based on the effective width method (EWM) in the Chinese code GB50018-2002 and the EWM and direct strength method (DSM) in the North American code , the local buckling bearing capacity of 66 cold-formed thin-walled steel (CFS) built-up box section short columns composed of C-section and U-section assembles connected by ST4.8 self-tapping screws was calculated by using the available results from experiment and finite element parametric analysis. The comparison between theoretical values and experimental and finite element (FE) values shows that the calculation results obtained from the EWM in GB50018-2002 and American Iron and Steel Institute (AISI) are conservative, while the results calculated by DSM are accurate but unsafe. Based on the above results, a superposition method was presented, and a theoretical calculation model for CFS built-up box-section short columns failed in local buckling under axial compression was proposed. Then，according to the variable parameter results of 48 finite element specimens, the combination coefficient（α） of the theoretical calculation formula was obtained from regression analysis, and the formula for calculating the ultimate bearing capacity of the built-up columns was obtained. Finally, the applicability and accuracy of the proposed design method were verified by comparing with the test data and the calculation results of Chinese and American codes. It also provides a simple method for calculating the bearing capacity of CFS built-up box-section short columns.