In the double- main cable suspension bridge system,the sags of the main cables change with the defor-mation of the pylon and the elastic extension of the main cables.Based on the assumption that the hangers of the top and bottom cables are inextensible,the vertical displacements of the top and bottom cables are equal when the de-formation of the pylon and the elastic extension of the main cables jointly play their roles.According to the above principle,this paper derives the expression of the longitudinal restraint stiffness of the bridge to the pylon as well as the relationship between the load transfer on the top and the bottom cables and the deformation of the pylon.Then,the solution method of the deflection of the load span under the uniform single- span load is investigated,and the corresponding analytical expression is obtained.Finally,all the formulas are verified by establishing a finite ele-ment model.It is found that the formulas accord well with the model.