Based on the overlapping area method and Haring theory, the formula for the shear post- yield stiffness of metal rubber bridge bearings with a circular cross- section was obtained in this study. The shear post- yield stiffness in the formula is proportional to the compressive stress and the bearing diameter and inversely proportional to the bearing height, where two coefficients α and β are to be determined. The relationship between the shear modulus and compression modulus of large- size metal rubber bridge bearing specimens was obtained through compression and compressive- shear tests, and coefficient α was approximately 1.3. Referring to the influence law of compressive stress, bearing diameter, relative density, and shape factor on the shear post- yield stiffness of the bearing in the shear test, the formula of the dimensionless coefficient β was evaluated. Furthermore, a practical simplification of the theoretical formulation was performed according to coefficients α and β, and the theoretical calculation results of the post- yield stiffness were compared with the experimental results. Results showed that the practical calculation formula, which agrees with the test results, is reasonably accurate. This formula can be used for the calculation of the post- yield stiffness of metal rubber bearings under compression- shear conditions.