Abstract: To investigate the torsional performance of composite box girders that have corrugated steel webs with cross-sections that can change,we establish the torsional stress equations and differential equations for these box girders based on Umansky’s second theory.To validate the theoretical formulas,we conduct finite element modelling.The study examined how arranging diaphragm beams affects the twisting performance of box girders featuring corrugated steel webs,under various loading conditions.In addition,and the transverse and longitudinal loading and deformation characteristics of the box girders were compared.The findings indicate that the mid-span diaphragm,which has poorer mechanical properties,has a more detrimental impact than the 1/3 span.The variable cross-section box girder’s torsional performance is not directly related to the diaphragm’s position.For force and deformation analysis,the mid-span cross-section should be the reference cross-section.Compared to the top plate,the stress on the bottom plate changes greatly when the spacing between the diaphragm plates decreases.The overall stress becomes stable when there are more than 8 diaphragm plates,meaning the spacing is less than 13m.Also,when the spacing of the diaphragm plates decreases,the stress on the bottom plate becomes stable too.Compared with the longitudinal buckling displacement,the setting of diaphragm has a more significant effect on the transverse buckling displacement of the bridge.Taking into account the strain and shape,the optimum number of diaphragms is between 6 and 8,spaced 10m to 20m apart.