<p>Extremely small-amplitude vibration environments are crucial to the stable operation of high-precision equipment, often causing complications. However, traditional vibration isolation techniques are inherently limited by their inability to achieve effective ultra-low-frequency vibration isolation. To overcome this, a six degrees of freedom (6-DOF) active vibration isolation system incorporating a quasi-zero stiffness (QZS) mechanism is proposed. This system actively isolates low-frequency, extremely small-amplitude vibrations in high-precision equipment by leveraging a hybrid control algorithm that integrates feedback proportional-integral-derivative and feedforward filtered-x least-mean-square controls. The QZS feature is driven by axial pre-compressed leaf springs. Additionally, a dynamic model of the QZS active vibration isolation system is developed to investigate 6-DOF transmissibility, and the results demonstrate that vibration isolation effects of more than 30.5 dB are achieved in all evaluated translational DOFs at the target frequency band. To validate the theoretical model, an experimental QZS-exhibiting active vibration isolation rig is evaluated. Overall, this study contributes to the systematic design of 6-DOF active vibration isolation systems for ultra-low-frequency, extremely small-amplitude vibration reduction in high-precision operating environments.</p>