This　paper　aimed　at　developing　a　theoretical　model　for　prestressed　composite　box　girders　with　corrugated　steel　webs　(CSW)　under　pure　torsion,　based　on　the　combined　action　softened　truss　model　(CA-STM).　The　equilibrium　equations　and　materials＇　constitutive　laws　were　modified,　considering　the　relationship　between　the　initial　stress　and　strain　generated　by　prestressing.　According　to　the　yield　state　of　the　CSW,　proper　shear　flow　conditions　were　also　introduced　to　describe　the　deformation　coordination　relationship　between　the　CSW　and　concrete　slabs.　The　effect　of　the　width　of　cantilever　slabs　on　the　torsional　performance　of　composite　box　girders　was　investigated.　An　optimized　algorithm　was　employed　to　solve　the　CA-STM,　resulting　in　a　more　efficient　and　stable　solution　procedure.　The　validity　of　the　model　was　confirmed　through　the　available　experimental　data.　Results　showed　that　the　prediction　accuracy　of　the　improved　CA-STM　for　depicting　entire　torsional　behavior　is　great,　when　considering　the　reasonable　shear　strain　relationship　and　modified　initial　torque　calculation　formula.