The negative stiffness properties of a mechanism and the quasi-zero stiffness properties of a vibration isolation system of the mechanism after being merged into the linear main spring were analyzed. The main harmonic, sub-harmonic and super- harmonic resonance amplitude-frequency response expressions of passive isolation system with quasi zero stiffness were derived. The results have shown that the negative stiffness region broadens as the ratio of the initial deformation of the spring to the length of the bar increases. The quasi-zero stiffness region of the vibration isolation system broadens as the ratio of the stiffness of the main spring to the total stiffness of the spring in the mechanism and the ratio of the initial deformation of the spring to the length of the bar in the mechanism increase. Compared with the amplitude-frequency characteristics of Duffing equation, the corresponding harmonic response amplitude-frequency characteristics of the passive vibration isolation system demonstrate great deviation. To the motion differential equations of this kind of passive isolation system, there is comparatively larger deviation when the product of the displacement exciting amplitude and the square of exciting frequency replace the exciting force amplitude of unit quality as the solution of Duffing equation.