A double-cable system has wide application prospects in multispan suspension bridges. Longitudinal restraint stiffness of a double-cable system to the tower was studied in this paper. Two formulas were derived to calculate the longitudinal restraint stiffness of a double-cable system to the tower and the load distribution between the cables, respectively. A mathematical model of a two-span double-cable suspension bridge was created to verify the formulas. The tower-top displacement and the cable force were obtained by formulas, and the results are in good agreement with the results calculated by the FEM. The effects of some important design parameters on longitudinal restraint were studied. Analysis shows that the restraint stiffness increases with the increasing of unit weight of the bridge, dead load distribution to the top cable, sag ratio of the bottom cable, and elastic stiffness of the cables. Effects of tower-top displacement and span length were also analyzed, and some suggestions were given to design a double-cable suspension bridge.