To describe the nonlinear mechanical behavior of the metallic dampers more accurately, this paper developed a novel restoring force model incorporating performance degradation. The force-deformation relationship of the restoring force model was divided into two stages: small and large deformation stages. During the small deformation stage, the combined hardening model was used to predict the restoring force of dampers as plastic deformation increased. The kinematic and isotropic hardening variables were adopted to reflect the Bauschinger effect and cyclic hardening of metallic dampers, respectively. When accumulative plastic deformation exceeded a specific value, the restoring force model entered the large deformation stage. During this stage, exponential and sigmoid functions were adopted to modify the combined hardening model to reflect the stiffness and strength degradation due to the significant out-of-plane deformation and fracture of metallic dampers, respectively Furthermore, test results from literature on slit dampers were compared with the simulation results from the proposed model and other commonly used models. The results indicate that the proposed model can accurately predict the nonlinear behavior of metallic dampers across the entire deformation range.