The Bi/Bi₂O₃-type oxygen sensor is extensively employed for oxygen monitoring in liquid lead-bismuth eutectic (LBE)-cooled reactors, yet its low-temperature measurement accuracy remains a critical bottleneck limiting engineering deployment. This study aims to extend the lower operating temperature limit of the sensor through synergistic optimization of the reference electrode and solid electrolyte. The effects of the Bi/Bi₂O₃ mass ratio, filling amount, and yttria-partially stabilized zirconia (YPSZ) electrolyte wall thickness on sensor performance were systematically investigated over 300–600 °C. Electrochemical impedance spectroscopy and finite element simulations (COMSOL Multiphysics^® 6.3, COMSOL Inc., Stockholm, Sweden) were used to elucidate the underlying mechanisms. The results show that the optimized sensor with a Bi/Bi₂O₃ mass ratio of 95:5, a filling amount of 10 g, and a YPSZ wall thickness of 1.5 mm extended the stable operating limit from 350 °C to 300 °C, achieving a relative electromotive force error of 3.13% at 300 °C and maintaining over 4000 h of drift-free service. The improved low-temperature accuracy is attributed to the reduced oxygen ion migration activation energy (0.48 eV) and lower bulk impedance of the thick-walled YPSZ after high-temperature activation. These findings provide a material optimization strategy and theoretical basis for wide-temperature-range, long-lifetime oxygen sensing in lead-based reactors.