Based on numerical model established with actual parameters of raw material and processes, micro mechanism of metal rubber’s nonlinear stiffness was researched. Numerical model was verified in some respects, such as forming force, tissue structure, size and quasi-static loading curve. Tissue structure of metal rubber was decomposed. Elastic micro structure model and its local coordinate system were set up. Spatial motion, elastic deformation on z direction, and elastic deformation in xoy plane of elastic micro structure were studied. Mathematic model of metal rubber’s nonlinear stiffness property was built. Micro mechanism of nonlinear stiffness was described. The numerical model can coincide with experimental results well and be reliable. In quasi-static loading process, elastic micro structure’s translation is the main spatial motion, and its spatial rotation is very small. The elastic deformation on z direction can be regarded as parallel connection of several curved beams. The elastic deformation in xoy plane can be treated as a curved beam’s bending deformation making the distance of two endpoints smaller. The mathematical model agrees with test results well and it contains basic structure parameters, micro feature parameters and nonlinear stiffness mechanism parameters. These parameters have clear physical significance and the mathematic model has a high accuracy. Micro mechanism of nonlinear stiffness can be explained well by theory models. The mathematic model has some guiding significance for metal rubber’s design.