By means of the analysis on vibration isolation mechanism of combined positive and negative stiffness, a new type ultra-low passive vibration isolation system was designed in this paper. The method of combined negative stiffness mechanism and positive stiffness spring was used in this isolator so that it possesses high supporting stiffness as well as ultra-low vibration stiffness, and its stiffness also has nonlinear characteristics, which resolved the problem of ultra-low frequency vibration isolation in the traditional passive vibration isolator. By analyzed the mechanical characteristic of this system, the expression of the stiffness of the isolator and the zero stiffness condition of system at the equilibrium position were deduced, which offer theory guide for the design of new type nonlinear ultra-low frequency vibration isolator.