This paper reveals the corresponding relationship between the geometric shape of the cable and the bending moment diagram of the simply supported beam under the vertical load of the suspension bridge cable, which can be called the bending moment diagram theory in terms of the elastic theory, deflection theory and finite displacement theory of the suspension bridge structure analysis and calculation. Because the bending moment diagram of simply supported beam is convenient, intuitive and clear in concept, it can greatly simplify the structural analysis and calculation of suspension bridge, and is an accurate analytical solution. The cable shape calculation software of suspension bridge is compiled according to the bending moment diagram theory, which can calculate the empty cable shape and the completed bridge shape under the action of suspender force, and introduces the software function and use method.