Abstract: 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.