The invention relates to a method for calculating stiffness characteristics of a high-strength third-grade plate spring having gradually changing stiffness and belongs to the technical field of suspension steel plate springs. The stiffness characteristics of the high-strength third-grade plate spring having gradually changing stiffness under different loads can be calculated according to structure parameters, elasticity modulus, rated loads and contact loads of main springs and auxiliary springs of the high-strength third-grade plate spring having the gradually changing stiffness. It can be known through a prototype test that stiffness characteristic calculation values under different loads are matched with prototype testing values, the provided method for calculating stiffness characteristics of the high-strength third-grade plate spring having the gradually changing stiffness is correct, and a reliable technical base is laid for design and calculation of the stiffness characteristics of the high-strength third-grade plate spring having gradually changing stiffness. By utilizing the method, reliable stiffness characteristic calculation values can be obtained, it can be ensured that the plate spring meets the suspension offset frequency design requirement, and the vehicle running smoothness can be improved. In addition, the design and testing costs can be reduced, and the product development speed can be improved.