In this paper, transmission line systems are modelled as multi-span cable structures. A force method model is proposed for analysing the static response of the multi-span cables with small sags. The accepted cable model reduces to two groups of differential equations (the equilibrium equations in y, z directions) and an integral equation (the compatibility equation). Substituting the differential equation solutions into the compatibility condition, the governing equation is obtained in terms of the tension component in chord direction. This equation has been named the force method equation (FME). In this way the infinite-degree-of-freedom dynamic system is effectively simplified to a system with only one unknown. Finally, one example is presented to illustrate the application of the proposed force method.