Abstract: The study aimed to evaluate the deformation capacity of box-section steel members. The strain ductility coefficient, a stain-based evaluation index, was proposed for box-section steel members. tests quasi-static were conducted oedsix box-section steed mombers, the experimental failure modes and hysteretic responses were investigated, and the ultimate deformation and plastic zome length of the specimers were extracted to obtain the strain dultility coefficients. The results show that the failure mode of all specimens involved local buckling deformation of both the flanges and webs, and the deformation processes of the two were coordinated with ench other. All specimens had full hysteresis curves with no significant pinching phenomenon, demonstrating good ductility and energy dissipation capacity. The ultimate strains and strain ductility coefficients of box sections decreased with the increase of the width-to-thickness ratio and axial compression ratio. Based on the test results, validated and reliable finite element models were established, and parametric studies were conducted to investigate the effects of flange width-to-thickness ratio, web height-to-thickness ratio, and axial compression ratio on the strain ductility coefficients. The results indicated that the strain ductility coefficients of box-section steel members exhibitsed a decreasing trend with the increase in flange width-to-thickness ratio, web height-to-thickness ratio, and axial compression ratio. The regression analysis was conducted to establish a calculation recommendation formula for the strain ductility coefficients. The validity of the formula was verified.