Abstract: In order to analyze the cable deflections of suspension bridges subjected to live loads, the gravity stiffness of the cable under its dead load should be considered. In this paper the new formulae for the tension force, deflection and gravity stiffness of the main cable under live loads are deduced by two equations, one is from a mathematical analogue between the cable configuration and the moment diagram of a simply supported beam, the other is from the basic work-energy relation principle，that is，the work done by a live load is stored as gravity potential energy. A typical example illustrates that the new formulae presented have better preciseness than the existing ones.