<p>The performance of traditional quasi-zero stiffness (QZS) systems in ultra-low frequency vibration isolation and energy harvesting is limited by constraints in achieving further stiffness reduction. The construction of high-order quasi-zero stiffness (HQZS) structures is a viable path to break through these barriers. Nevertheless, current HQZS implementations are limited in number, typically configuration-dependent, and difficult to design and extend. This paper highlights the essential role of high-order optimization in further reducing equivalent stiffness and proposes a coefficient compensation approach by focusing on Taylor expansion coefficients of the restoring force. This methodology pioneers a new design paradigm through the strategic combination of structures. A bio-inspired negative stiffness structure is designed to upgrade a conventional three-spring QZS system into a 9th-order QZS system. Theoretical, simulation, and experimental results demonstrate that high-order optimization significantly reduces the equivalent stiffness of the energy harvesting vibration isolator, leading to greatly enhanced performance in both ultra-low frequency vibration isolation and energy harvesting under harmonic and random excitations. This study offers valuable insights for designing high-order QZS systems and inspires new ideas for ultra-low frequency vibration control and energy harvesting technology.</p>