A nonlinear enhanced bellows-type hydraulic inerter-based antiresonance vibration isolator is proposed for low-frequency line spectra vibration isolation. The nonlinear dynamic model of the enhanced system and the quasi-static model with bistable negative stiffness are established. The influence of parameters such as geometric dimensions and elastic coefficients on the nonlinear stiffness characteristics of the system is studied. It is found that the structure with negative stiffness enhancement only regulates the extent of nonlinear stiffness without changing the load-bearing capacity or static deformation. Subsequently，an estimation analysis of vibration isolation performance is conducted. The vibration transmissibility of the degraded linear system under force excitation is studied，and the effects of non-dimensional parameters，including the inertial mass ratio，effective area ratio，and damping ratio，on the transmissibility characteristics are analyzed. The dynamic response is solved by using the averaging method，and the analytical solution steps for the transmissibility of the nonlinear enhanced system are given based on the equivalent linearized stiffness. The analytical results are validated by comparing them with numerical simulation results，showing small relative errors，and thus can be used for design purposes. A comparative study is conducted on the transmissibility characteristics of the nonlinear negative stiffness enhanced hydraulic inerter-based vibration isolation system. The results indicate that the introduction of a bi-stable negative stiffness can lower the resonance and anti-resonance frequencies of the isolation system. By designing appropriate inertial mass parameters，it is possible to achieve superior wideband isolation effectiveness in the low-frequency range.