Abstract: Considering the extension and bending deformation of cable in deep sea, the 6 generalized displacements of nodes is described by rectangular coordinates and their tangent vectors. The nonlinear stiffness matrix is obtained based on the rod theory considering the geometric nonlinearity and the effect of water dynamic. The element can be called by general finite element software. A spar platform with 4 mooring cables in 318m depth is analyzed, the modes and natural frequencies of the mooring system are calculated and the tension response of cable system is obtained. The result shows that the dynamics properties of mooring cable in deep sea can be modeled with user defined element。