The　mechanical　properties　of　concrete-filled　steel　tubular　(CFST)　columns　with　different　sizes　under　torsional　load　were　studied,　aiming　to　further　explore　the　pure　torsional　failure　mechanism　and　size　effect　behavior　of　CFST　columns.　First,　a　3D　meso-scale　numerical　model　of　CFST　is　established　and　compared　with　the　existing　experimental　data.　Second,　the　torsional　failure　mechanism　of　CFST　column　under　different　section　sizes,　constraints　(confinement　coefficient),　and　cross-sectional　shapes　is　considered.　Results　show　that:　(1)　the　nominal　torsional　strength　of　CFST　columns　presents　a　size　effect,　and　the　columns　with　square　cross-section　present　stronger　size　effect　than　the　ones　with　circular　shape;　(2)　with　the　increase　of　the　confinement　coefficient,　the　size　effect　of　the　nominal　torsion　strength　is　weakened.　Finally,　considering　the　quantitative　influence　of　the　size　effect,　a　formula　for　calculating　the　pure　torsional　capacity　of　CFST　columns　was　developed.