In order to reveal the longitudinal deformation characteristics and mechanism of suspension bridges under vertical loads， the deformation characteristics of the cable under vertical loads were studied based on its deformation theory， and the analytical solutions of vertical and longitudinal displacement were obtained. On this basis， the longitudinal deformation characteristics and the longitudinal displacement of the stiffening girder for the traditional suspension bridge and the suspension bridge with central buckles are analyzed under vertical loads， the longitudinal restraint mechanism of the central buckles is subsequently revealed， and the simplified formulas to estimate the longitudinal displacement of the stiffening girder under vertical loads are deduced， respectively. Finally， the accuracy of the proposed displacement calculation formulas is verified by an example. The results show that the vertical and longitudinal displacement are coupled to each other due to the geometric nonlinear characteristics of the main cable. The main cable undergoes asynchronous longitudinal displacement， while the stiffening girder mainly moves longitudinally as a rigid body similar to the pendulum under the action of asymmetric vertical loads. The main cable， the stiffening girder， and the central buckles form a "rigid domain" area in the middle of the span as the existence of the central buckle. As a result， the longitudinal displacement of the stiffening girder decreases. The proposed method agrees well with the finite element numerical solution under different vertical load cases.