Abstract: To address the challenge of limited adaptability to varying loads in existing quasi-zero-stiffness isolators, this study designed a load-adjustable piezoelectric quasi-zero-stiffness isolator. This isolator primarily comprised a piezoelectric patch with an externally connected negative capacitance shunt circuit and a cosine-shaped curved beam. The nonlinear stiffness of the curved beam could be adjusted, by modifying the negative capacitance value, allowing for control over the isolator’s load-bearing capacity. Initially, by adopting modal superposition and minimum total potential energy principle, the force-displacement relationship of the piezoelectric curved beam was derived. The derived relationship accuracy was validated through comparison with finite element results. Subsequently, the piezoelectric curved beam was coupled with a U-shaped stiffer wall to form the isolator. Both the simplified analytical model and the comprehensive finite element model of the isolator were established to investigate its isolation characteristics under various loads. The results indicate that the regulation of the modulus of piezoelectric materials through negative capacitance circuits can generate the quasi-zero-stiffness characteristics under different loads, achieving excellent low-frequency vibration isolation performance.