<p>In this paper, a coupled mismatched time-lag controlled quasi-zero static stiffness vibration isolator (QZS-VI) system is investigated to enhance its vibration isolation performance. Firstly, a mathematical model of the controlled QZS–VI system is established, and the steady state response is solved analytically using the averaging method and further verified by the Runge–Kutta numerical method. Subsequently, the stability of the system is analysed in depth, and the effects of Hopf bifurcation and saddle-node bifurcation on the dynamic characteristics of the system are explored. The modulation effects of the dual time-lag parameters on the amplitude-frequency characteristics, peak amplitude and resonant frequency are further investigated, and it is found that the proper selection of the time-lag parameters can effectively control the emergence and evolution of the frequency islands. In addition, the effect of double time-lag control on the force transfer rate of the system is analysed, and the results demonstrate that an appropriately mismatched time-lag control can attenuate the resonance peak and broaden the vibration isolation frequency band. Finally, the equivalent damping effect of displacement–velocity feedback control is discussed, and it is indicated that different time-lag parameters can optimise the vibration isolation performance of the system. The results show that mismatched time-lag control can significantly improve the vibration isolation effect of the QZS–VI system, which provides theoretical guidance for the optimal design of nonlinear vibration systems.</p>