Under　the　action　of　earthquake,　concrete-filled　steel　tube　(CFST)　columns　are　prone　to　compression-torsion　failure　due　to　torsion　and　the　torque　action　will　aggravate　the　destruction　of　CFST　column.　In　this　paper,　a　mechanical　analysis　model　and　numerical　simulation　method　for　CFST　columns　are　established,　which　can　take　into　account　both　the　concrete　heterogeneous　characteristics　and　the　concrete/steel　tube　contact　behavior,　and　then　the　failure　behavior　and　size　effect　of　CFST　columns　under　the　combined　action　of　compression　and　torsion　are　simulated　and　studied.　The　effect　of　structural　size,　axial　compression　ratio,　steel　ratio　and　section　shape　on　failure　mode,　nominal　torsional　strength　and　size　effect　behavior　of　CFST　columns　are　analyzed.　The　results　show　that:　With　the　increase　of　axial　compression　ratio,　the　torsional　capacity　of　the　CFST　column　first　increases　and　then　decreases,　and　the　torsional　capacity　of　the　member　reaches　the　maximum　value　when　the　axial　compression　ratio　is　about　0.4.　The　size　effect　of　CFST　columns　under　compression　and　torsion　is　found　to　a　certain　extent;　according　to　the　simulated　working　conditions　in　this　paper,　the　nominal　torsional　strength　decreases　by　24%;　in　addition,　with　the　increase　of　axial　compression　ratio,　the　size　effect　of　nominal　torsional　strength　of　members　decreases　first　and　then　increases.　Based　on　the　numerical　test　data　of　this　paper　and　the　author’s　previous　work,　a　formula　for　size　effect　of　nominal　torsional　strength　of　CFST　columns　considering　the　influence　of　axial　compression　ratio　and　steel　content　is　proposed.　©　2024　Chinese　Academy　of　Sciences.　All　rights　reserved.