Abstract: Taking high static low dynamic stiffness (HSLDS) vibration isolator with rhombic negative stiffness mechanism (rhombic HSLDS isolator) as research object, the equivalent friction model of negative stiffness mechanism was established by virtual work method, and the kinetic equation with factors of negative stiffness mechanism mass and friction was established by Lagrange method. The kinetic equation was solved by harmonic balance method (HBM), the influences of negative stiffness mechanism mass and friction on vibration isolation and optimization measures were analyzed, and the rationality of theoretical model was verified by the prototype. The experimental results show that the mass and friction of negative stiffness mechanism have adverse effect on vibration isolation and should be reduced. Connecting the shorter rod side of negative stiffness mechanism to the end of load platform can reduce influence of the negative stiffness mechanism mass on vibration isolation performance in higher-frequency range. Under condition that stiffness parameters of isolator and contact parameters of hinged pairs are fixed, and the stiffness and friction are satisfied simultaneously, the vibration isolation performance in low-frequency range can be optimized by increasing the rod length difference, and can reduce the influence of negative stiffness mechanism friction on vibration isolation in high-frequency range.