Purpose <p>The aim of this paper is to investigate an adjustable quasi-zero-stiffness (QZS) energy harvesting system, with a special focus on performance enhancement in ultra-low frequency vibration energy harvesting. By proposing a system based on the energy harvesting function of the gear structure, the performance degradation caused by the sharp increase in stiffness of the conventional QZS isolator when the displacement is increased is solved, which in turn improves the efficiency of energy harvesting.</p> Methods <p>Runge-Kutta method is used to obtain multicolor plots of the root-mean-square (RMS) values of the induced voltages, and the system’s dynamic properties and response behavior are examined. In addition, the effect of initial value changes on the system trajectory and energy harvesting efficiency is explored by mapping the basin of attraction of the coexisting solutions of the system.</p> Results <p>The results show that the proposed adjustable QZS system exhibits good adaptability under different external environments. The variation of the RMS value of the induced voltage demonstrates the advantages of the system for ultra-low-frequency vibration energy harvesting. In addition, the system trajectory is significantly affected by the initial values, some of which lead to stable periodic states while others cause chaotic or unstable motions, which is directly related to the energy harvesting efficiency.</p> Conclusions <p>The adjustable QZS energy harvesting system proposed in this paper effectively improves the energy harvesting efficiency under ultra-low-frequency vibration and overcomes the limitations of conventional QZS isolators in terms of stiffness and performance. The close connection between the effective value of the induced voltage and the nonlinear dynamic behavior is confirmed by analyzing the effect of the initial value on the trajectory of the system through the basin of attraction diagram.</p>