This paper investigates the potential of utilizing frequency-varying negative stiffness properties of the inerter in vehicle suspensions to improve the comprehensive dynamic performance. Firstly, the dynamic characteristics of the spring and the inerter are analyzed to evaluate the feasibility of integrating them into vehicle suspensions. Secondly, a quarter car model incorporating the nonlinear stiffness features of an air spring is established, and a vehicle semi-active control strategy for the air ISD (inerter-spring-damper) suspension based on the frequency-varying negative stiffness of the inerter is proposed. Thirdly, in order to implement this strategy effectively, this paper builds an uneven road surfaces estimator based on the DKF-UI (discrete Kalman filter with unknown input) and an uneven road surfaces frequency identifier based on the first order-zero crossing algorithm. Finally, the superiority of the proposed suspension system is verified by simulations, and the results reveal that with respect to the passive air suspension, the peak values of the gain of the body acceleration, the suspension working space and the dynamic tire load are reduced. The vehicle semi-active air ISD suspension exhibited a reduction in body and wheel frequency regions of 68.7%, 51.0%, 67.6% and 5.1%, 9.9%, 7.9%, respectively. Under a segment sinusoidal road input, the RMS value of three performance indicators exhibited a reduction in 1.4 Hz and 11 Hz of 79.2%, 57.8%, 77.7% and 6.8%, 15.4%, 5.8%, respectively. Consequently, the vehicle semi-active air ISD suspension proposed in this paper has significant performance improvement compared to the passive air suspension.