Concrete-filled　steel　tubular　(CFST)　short　columns　under　the　action　of　wind　and　seismic　load　often　endure　high　shearing　force　and　show　brittle　shear　failure,　which　may　exhibit　obvious　size　effect.　In　this　study,　a　three-dimensional　meso-scale　simulation　method　that　can　consider　both　the　concrete　heterogeneity　and　the　contact　behavior　between　concrete　core　and　steel　tube　was　established　to　investigate　the　failure　behavior　and　size　effect　of　square　CFST　columns　under　combined　lateral　and　axial　loads.　After　verification　of　the　meso-scale　model,　simulation　tests　were　carried　out　to　discuss　the　influence　of　shear-span　ratio,　axial　compression　ratio　and　steel　ratio　on　the　failure　modes,　the　nominal　shear　strength,　the　ductility　and　the　size　effect　of　square　CFST　columns.　Results　indicate　that,　when　the　shear-span　ratio　increases　from　1.0　to　3.0,　the　failure　mode　of　CFST　columns　turns　from　brittle　shear　failure　to　ductile　bending　failure,　the　nominal　shear　strength　decreases　and　ductility　increases,　and　the　size　effect　in　shear　would　be　weakened.　The　nominal　shear　strength　increases　and　the　column　shows　better　ductility　with　the　increasing　axial　compression　ratio　less　than　0.4,　while　the　nominal　shear　strength　decreases　and　the　columns　present　obvious　brittleness　with　the　increase　of　axial　load　larger　than　0.4.　Moreover,　as　steel　ratio　increases　from　0.05　to　0.15,　the　nominal　shear　strength　and　ductility　of　CFST　columns　increase,　and　the　size　effect　on　nominal　shear　strength　cannot　be　weakened.　At　last,　a　theoretical　formula　that　can　quantitatively　show　the　influence　of　shear-span　ratio　on　size　effect　in　shear　strength　was　proposed　to　predict　the　shear　capacity　of　square　CFST　columns.　The　available　test　data　and　the　simulation　results　illustrate　the　rationality　and　applicability　of　the　theoretical　formula.　©　2020　Elsevier　Ltd