Based on 1T15M-2 reversible sleeve air spring made by Firestone corp., the vertical stiffness of the air spring was analyzed by using the theoretical method. A quarter accurate finite element model of the air spring was established in ANSYS, the vertical stiffness of the air spring was studied by using the non-linear finite element method (FEM), and the simulation vertical stiffness under three different pressures was obatined. The vertical stiffness obtained by simulation and theoretical calculation respectively was almost consistent with the test value, which showed the valid of the method. Theoretical analysis showed that the maximum vertical stiffness effected by changing rate of effective areas was more than half of total vertical stiffness, which can not be neglected. The method was efficient to calculate vertical stiffness of air spring with arbitrary shape of piston.