A quasi-zero stiffness isolator with cam-roller mechanism was proposed, and the piecewise nonlinear restoring force was obtained by static analysis. The equation of motion of the vibration isolation system was established, and the first approximation of primary resonance was achieved by Harmonic Balanced method. Moreover, the parametrical regions, where the cam always keeps touch with the roller, and the force transmissibility were given based on approximate solutions. Finally, the experiment was carried out to evaluate force transmissibility under excitations with various frequencies, which was compared with that of the corresponding linear vibration isolation system. The results show that the cam-roller mechanism with negative stiffness lowers the starting isolation frequency and enhances the isolation efficiency, and suppresses the resonant response, leading to an excellent low-frequency vibration isolation performance. Also shown is that, under the condition of keeping touch between the cam and the roller, the larger the excitation amplitude is, the more likely to reduce the system’s stiffness the negative-stiffness mechanism is, to achieve more notable vibration isolation performance.