Considering that the load weight changes can affect the performance of vibration isolation, an adjustable nonlinear pneumatic vibration isolator with the favorable characteristic of quasi-zero stiffness (QZS) was established. The QZS isolator was devised by connecting a vertical double-chamber adjustable pneumatic spring with four symmetrical initial horizontal single-chamber adjustable pneumatic springs. Analytical result revealed a unique relationship between the design parameters of pneumatic springs of the QZS systems. The adjustable QZS isolator could maintain the characteristic of quasi-zero stiffness by adjusting air pressure when the mass loading changed. In vibration analysis, the force transmissibility derived by the harmonic balance (HB) method was used for the performance evaluation of the vibration isolation systems. The adjustable QZS isolator outperforms a corresponding linear system, especially in low-frequency vibration isolation performance.