Light of the mechanical equipment of heavy-duty and low dynamic frequency, the pneumatic quasi-zero stiffness nonlinear vibration isolation technique is proposed. The presented technique employs negative gas pressure to produce the static bearing capacity, and the low dynamic stiffness is provided by the elastic bellows structure. Hence, the given isolator possesses the high-static-and-low dynamic (HSLD) stiffness. First, the structure and the working principle of the smooth type isolation device is introduced. Then, the isolator’s nonlinear stiffness model is derived based on the gas state equation. Furthermore, the influence of physics parameters including gas proportion and gas pressure on the elastic restoring force is discussed. The results show that a proper gas proportion can tune the low dynamic stiffness of the isolator, and limit the maximum amplitude. Moreover, the effects of overloading acceleration on the inherent frequency of the isolation system are examined. It is found that overloading acceleration diverges the static equilibrium position away from its original point. As a result, the corresponding change of the natural frequency of the linearized system deteriorates the performance of vibration isolation which should be taken into consideration in engineering.