Low-frequency line spectrum is a crucial characteristic signal for detecting, tracking, and identifying targets in modern underwater acoustic warfare. The quasi-zero stiffness vibration isolator (QZS VI) possesses both high static stiffness and low dynamic stiffness, which can effectively suppress low-frequency vibrations. A novel quasi-zero stiffness vibration isolator with load adjustment (QZS VI-LA) is proposed. Firstly, a magnetic force analytical model of the ring magnet negative stiffness mechanism (NSM) is established using the filament method. A quasi-zero stiffness matching analysis of the QZS VI-LA system is conducted, and a non-ideal load adjustment mechanism (NLAM) is designed. Secondly, the dynamic equations of the QZS VI-LA system with the flexible foundation are formulated. The dynamic characteristics of the system are analyzed using the harmonic balance method and the pseudo-arc length method. Finally, a prototype of the QZS VI-LA is constructed, and experimental results demonstrate that the QZS VI-LA system has excellent low-frequency vibration isolation performance and load adjustment capability.