Abstract: The cold-formed steel quadruple-limb built-up box-section column is one type of commonly used composite member, adopted by multi-storey cold-formed thin-walled steel structure buildings to mainly bear compressive concentrated loading. Numerous experimental studies have been carried out to study its ultimate bearing capacity, but there is still a lack of reliable design method, which is not convenient for engineering application. Based on the existing experimental research results, the numerical analysis model is established and verified. The parameter analysis is carried out on the verified numerical model, and the bearing capacity of cold-formed thin-walled steel four limb box section columns with different dimensions and structures are analyzed. Based on the numerical and energy method, the critical loads for both local and overall buckling of built-up columns are calculated respectively. Combined with the direct strength method in American code, the corresponding calculation method of ultimate bearing capacity is proposed. The results show that the modified method proposed has higher accuracy and safety than that utilized in American relative code.