In　this　study,　the　mechanical　behavior　of　geometrically　similar　square　concrete　filled　steel　tubular　(SCFST)　columns　under　axial　compression　was　investigated　using　numerical　modeling.　The　effects　of　the　confinement　effect　and　structural　size　on　the　axial　compressive　behavior　of　the　SCFST　column　were　studied.　A　3D　mesoscale　model　of　the　SCFST　column　under　axial　compression　was　established　and　verified　using　available　test　results.　The　failure　mode　and　the　nominal　axial　stress　-axial　strain　curve　were　then　presented　and　analyzed.　The　effects　of　the　steel　ratio　and　section　size　on　the　failure　mode　of　the　SCFST　column　were　not　obvious.　As　the　section　size　of　the　SCFST　column　increases,　the　peak　axial　stress　and　corresponding　peak　axial　strain　show　a　descending　trend.　Specifically,　as　the　section　size　changes　from　100　mm　to　800　mm,　the　peak　axial　stress　decreases　by　46.2%　and　23.8%　corresponding　to　the　steel　ratios　of　0.0　and　0.21.　With　an　increase　in　the　steel　ratio　increases　and　enhanced　confinement　effect,　the　influence　of　the　size　effect　on　the　peak　axial　stress　decreases.　For　the　outer　steel　tube,　the　vertical　stress　increases　and　the　hoop　stress　decreases　with　the　increasing　section　size,　leading　to　the　weakened　confinement　effect.　Subsequently,　an　empirical　formula　considering　the　size　effect　was　proposed　to　calculate　the　axial　bearing　capacity　of　SCFST　columns　with　different　structural　sizes　and　confinement　effects.　The　proposed　empirical　equation　was　validated　using　collected　test　results.　This　equation　can　be　applied　to　predict　the　axial　load　bearing　capacity　of　SCFST　columns　with　different　structural　sizes　and　steel　ratios,　especially　for　large　structural　sizes.