Considering the broad application prospect of high-strength steel (HSS) in engineering structure, and the limitations present in existing studies on interactive buckling of HSS welded thin-walled box section long column, test and finite element analysis (FEA) were used to investigate the interactive buckling behaviour exhibited by 12 specimens fabricated from Q420-Q960 steels. In the detailed analysis, various aspects were considered, including the failure mode, axial deformation, interaction between overall and local buckling deformation, and the ultimate bearing capacity. It was observed that the utilization rate of steel strength diminished with an escalation in both the plate width-thickness ratio and steel strength. An increase in the width-thickness ratio correlated with an earlier onset of local buckling, and the influence of steel strength was found to be negligible. Taking into account the influence of slenderness ratio and steel strength, a novel calculation formula for determining the limit of the plate width-thickness ratio for columns undergoing interactive buckling was derived. The limit of the plate width-thickness ratio experienced a decrease with an increase in steel strength, and the influence of the column slenderness was minimal. Subsequently, the ratios of calculated to simulated ultimate bearing capacities for non-compact section long columns were evaluated using Eurocode 3, ANSI/AISC 360-22, and JGJ/T 483-2020. Additionally, a novel calculation method for the ultimate bearing capacity of Q420-Q960 steel welded thin-walled box section long column was introduced, grounded in the direct strength method.
