In order to study the distortional buckling behavior of cold-formed steel（CFS） built-up box-section columns，firstly，9 C-section columns，9 U-section columns，and 21 built-up box-section columns composed of C-section and U-section were investigated by the axial compression tests and numerical simulation. The buckling characteristics and mechanical behaviors of the columns were investigated. On this basis，a set of hypothesis models were proposed to study the influence of screw spacing on the deformation，the ultimate capacity，and the relationship between the screw spacing and the half-wavelength（λc） of CFS built-up box-section columns. The results show that：1） The number and half-wavelength of distortion buckling are different for the specimens with different screw spacing. 2） When the screw spacing is less than 0.9λc，the ultimate capacities of built-up box-section columns are greater than the sum of the ultimate capacities of C-section and U-section columns，that is，the splicing effect of 1+1>2. 3） When the screw spacing is greater than 0.9λc，the ultimate capacity of the built-up box-section column is gradually close to the sum of the ultimate capacities of the C-section and U-section columns，i.e. 1+1≈2. In order to calculate the ultimate capacities of CFS built-up box-section columns，an approach to predict the elastic critical load of distortion buckling was proposed based on the direct strength method（DSM） in American code，and then the calculated results were applied to the DSM to obtain the ultimate capacity of CFS built-up box-section columns. The results obtained from the calculation method are in good agreement with the experimental and numerical simulation results，which suggests the accuracy and applicability of the proposed method in this paper.