Inertial devices can convert linear motion into high-speed rotation to amplify the physical mass of the system. Combining a conventional tuned mass damper (TMD) with an inertial device results in a tuned inertial mass damper (TMDI). Previous studies by many scholars have shown that the TMDI system has certain advantages in controlling the vibration response of structures. In order to make the TMDI system fully exploit its vibration reduction potential, this paper proposes an accurate optimal parameter design formula for the single-degree-of-freedom TMDI system under harmonic load excitation. In order to verify the effectiveness of the optimal parameter design formula, a comparative study with the traditional tuned mass damper (TMD) was also carried out. The results show that when a single-degree-of-freedom structure is subjected to random seismic excitation, the parameters obtained by the TMDI optimal parameter design method proposed in this paper can effectively realize the vibration reduction potential of TMDI. When the traditional tuned mass damper (TMD) is used to control the vibration response of structures under non-stationary random seismic excitation, its vibration reduction effect is not ideal, but after the TMDI system is set with the optimal parameters, it can achieve the expected vibration reduction performance.