Abstract: The vertical stiffness of an air spring cannot be accurately predicted, because the existing finite element software cannot calculate the polytropic process of air. A solution for simulating the polytropic process of air springs was proposed in order to solve this problem. The finite element formulation was deduced based on virtual work, and the elasticity of air was deduced by introducing the polytropic index based on an ideal gas status equation. A pneumatic element was developed based on the user-defined element UEL, which is provided by ABAQUS software. Compared with the experimental results, we find the proposed method predicts the vertical stiffness of an air spring very precise under different frequencies, loads and additional reservoir volumes, which validates the proposed method.