Based on a generalized cable-girder model, it was found that the torsional stiffness degradation of the cable system was the essential reason leading to aerostatic torsional divergence of long-span suspension bridges. The effects of the deformations of the main cables and the bridge tower on the stiffness degradation of suspension bridges were investigated. Theoretical analysis indicates that vertical deformation of the main cable is critical to the torsional stiffness of the whole system. Provided that the vertical deformation of any cable reaches a critical value, the cable will loosen up to a non-stress state, which causes a sudden drop in torsional stiffness of the system. Therefore, it is stated that the sudden drop in stiffness due to the vertical deformation could be the key reason for the aerostatic torsional divergence of a long-span suspension bridge. Moreover, the study also shows that lateral deformation of the main cable and the horizontal relative deformation between the two tower-tops postpone the stiffness degradation, and hence enhance the critical vertical deflection. In addition, it is shown that the effect of turbulence on torsional divergence is non-negligible, and the turbulence significantly decreases the aerostatic torsional stability of bridge structures. Numerical results of the Xihoumen Suspension bridge performed by nonlinear finite element simulation could be explained tentatively by the theoretical viewpoints proposed in this paper.