In this paper, a novel type of passive vibration isolation mechanism with quasi-zero-stiffness is proposed and designed, in order to solve the problem of low frequency vibration of rehabilitation robot. It is achieved by two symmetrical tension springs with a negative stiffness characteristic in parallel with a linear positive stiffness spring. Through the study of static characteristics,the relationship between the force displacement and the stiffness displacement of the vibration isolation mechanism isestablished, it is concluded that the parameters of the mechanism have quasi-zero-stiffness characteristics at the equilibrium position. The dynamic characteristics, nonlinear dynamic equations in harmonic power and the harmonic displacement excitation are studied, respectively. The influence of mechanism parameters and the excitation on the system force transfer rate and the displacement transfer rate are analyzed using a harmonic balance method. Finally, the curves of the input and output are compared with or without the negative stiffness through experiments. The results show that in the case of negative stiffness mechanism, the vibration isolation system has a certain range of quasi-zero-stiffness characteristics and the low frequency vibration isolation performance is better, which can achieve the effect of high static and low dynamic stiffness. It is innovative and instructive for the low frequency vibration isolation of robot.