Abstract: This study proposes a Diamond Mechanism Quasi-zero Stiffness Base Isolation System (DQZS-BIS). By leveraging the nonlinear deformation of the diamond mechanism, DQZS-BIS enjoys a special nonlinear stiffness property. Under the small excitation, the stiffness of the DQZS-BIS is about 0; while under the large excitation, the stiffness of the DQZS-BIS is extremely large. This special nonlinear stiffness can be used to enhance the seismic isolation performance of BIS under different excitations. Theoretically, the equivalent single-degree-of-freedom system of DQZS-BIS is given. Harmonic balance method is used to analyze the nonlinear dynamic response of DQZS-BIS in frequency domain. Times-history simulations are performed to study the seismic isolation performance of the proposed DQZS-BIS. Compared with the L-BIS, the proposed nonlinear DQZS-BIS enjoy better isolation performance. Under small peak ground motions, the stiffness of the DQZS-BIS is smaller, which prolongs the isolation period and suppresses the acceleration response of the supper-structure; under large peak ground motions, the stiffness of the DQZS-BIS is larger, which inhibits the deformation of the isolation layer.