While single network hydrogels show limited extensibility and low strength, double-network hydrogels benefit from significantly high stretchability and toughness due to their reinforcing mechanism of combining two soft and rigid networks. Here, a micro-mechanical model is developed to characterize the constitutive behavior of DN hydrogels in quasi-static large deformation. In particular, we focused on describing the permanent damage in DN gels under large deformations. Irreversible chain detachment and decomposition of the first network are explored as the underlying reasons for the nonlinear inelastic phenomenon. The proposed model enables us to describe the damage and the way it influences the micro-structure of the gel. The model is validated with uni-axial loading and unloading experiments of DN gels. The proposed model contains a few numbers of material constants and shows a good agreement with cyclic uni-axial test data.