This　paper　addresses　the　problem　of　plastic　hinge　length　in　the　evaluation　of　ductility　performance　of　square　concrete　filled　steel　tubular　(CFST)　column　that　was　largely　used　in　earthquake-resistant　structures.　Physical　tests　were　conducted　on　a　series　of　square　CFST　stocky　columns　having　different　cross-section　widths　(B　=　200　mm　&　SIM;800　mm).　The　curvatures　of　each　column　were　measured,　serving　as　the　basic　building　blocks　for　establishing　the　linkage　between　the　plastic　hinge　length　and　the　cross-section　width.　From　the　experiments,　it　was　found　that　the　failure　region　of　all　columns　is　concentrated　in　the　bottom　of　the　CFST　columns,　which　is　located　in　the　plastic　hinge　region.　The　plastic　hinge　length　of　square　CFST　stocky　column　increases　linearly　with　the　increase　of　the　cross-section　width.　Meantime,　an　analytical　approach　is　developed　based　on　the　equivalent　plastic　hinge　model　and　cross-section　analysis,　by　which　the　analytical　formulas　of　plastic　hinge　length　were　proposed.　The　important　parameters　involving　axial　ratio,　strengths　of　concrete　and　steel　plate,　slenderness　ratio　as　well　as　the　width-to-thickness　ratio　on　the　plastic　hinge　length　of　square　CFST　column　were　studied.　The　results　indicate　that　the　plastic　hinge　length　tends　to　increase　with　the　increase　of　the　axial　load　ratio,　however,　decreases　with　the　increase　of　the　aspect　ratio　and　the　compressive　strength　of　concrete.　No　significant　effects　of　the　width-to-thickness　ratio　and　the　steel　strength　were　found　on　the　plastic　hinge　length　of　the　square　CFST　column.