Abstract: Amathematical model for elastic rod theory is applied according to the structure characteristic of the secondarysuspension systemoftilting locomotive, which is composed of screwflexicoil spring and rubber spring. The relationship between spring load and deformation is conducted in this paper. The authors also proposed a newestimation method for the stability of combined spring system-lateral stiffness method, which can avoid negative stiffness when this kind of suspension system is applied to engineering. It is shown that the newmethod have more accuracy and adaptability than other presented methods.