The base isolation technique is widely applied in building structures to mitigate structural seismic responses, while the acceleration response of the main structure is reduced at the cost of isolator deformation in the isolation system. A hybrid isolation system comprising a tuned inerter damper (TID) and an isolator can remarkably reduce the displacement response of the isolation layer. Similar to a traditional tuned mass damper (TMD), a TID comprises an inerter, tuning springs, and damping elements. Therefore, the design formula of the TMD system can be directly used to determine the optimal parameters of TID. First, two design formulas of TID and TMD were derived and analyzed on the basis of motion equations of the single-degree-of-freedom system with additional TID. Then, the precondition and applicability of the two formulas were comprehensively discussed. Finally, the feasibility and effectiveness of the designed TID were val-idated by the benchmark model of the base isolation system. Numerical simulation results illustrate that the designed TID can significantly reduce the displacement and base shear responses of the base isolation structure without increasing the absolute acceleration response of the main structure.