Various of cable elements are used in the numerical analysis of cable structures. According to the mechanical characteristics of highline cable system of alongside replenishment at sea, two cable elements are investigated: catenary element, which is the exact solution, and parabolic element, which is the approximate solution but simpler to calculate. The equilibrium equations of each cable element are introduced, and the expressions of tangent stiffness matrix are derived. The tangent stiffness matrix of catenary element is calculated through Newton interactive method.