Applying　generalized　force　and　strain　into　element　analysis,　the　elastic-plastic　stiffness　matrix　is　readily　separated　into　elastic　and　plastic　stiffness　matrices　in　which　the　material　nonlinearity　of　a　beam　element　are　taken　into　account.　The　total　deformation　of　the　beam　element　can　be　accordingly　obtained　by　imposing　plastic　deformation　on　elastic　deformation,　and　the　internal　force　can　be　obtained　by　multiplying　the　slope　of　elastic　strain　energy　(elastic　stiffness　matrix)　with　the　deformation,　so　the　unbalanced　forces　after　deformation　can　be　calculated　more　quickly　and　accurately　during　iterative　analysis.　Based　on　the　above-mentioned　procedure　and　so　called　fiber-model　beam　element,　the　primary　finite　element　formula　is　derived　for　concrete　filled　steel　tubular　structures.　This　formula　is　implemented　in　a　geometrically　nonlinear　finite　element　analysis　computer　program　NL_Beam3D　based　on　the　3-D　beam　element,　to　solve　dual-nonlinearity　problem.　The　proposed　method　and　procedure　are　verified　in　a　case　to　describe　nonlinear　behaviors　of　the　concrete　filled　steel　tubular　structure　accurately.