Abstract: Accurate sensors for measuring oxygen concentration in liquid lead-bismuth eutectic (LBE) alloys are essential for ensuring the safe operation of LBE systems and extending the service life of equipment. The Lanthanum-Strontium-Cobalt-Iron (La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ, LSCF)/ air oxygen sensor attracted considerable attention in the field of LBE oxygen concentration measurement due to its wide temperature range adaptability and high accuracy. This type of oxygen sensor employs LSCF powder as the reference electrode. In this study, the performance of oxygen sensors assembled with LSCF reference electrodes of 200 nm, 0.6 μm, and 3 μm particle sizes is evaluated in oxygen-saturated LBE within the temperature range of 200~550 °C. The results show that the LSCF/air sensor with a 200 nm reference electrode powder exhibits an output potential deviation of only 1.62% from the theoretical value across the entire temperature range. Additionally, the maximum average fluctuation of the signal is 2.21 mV, and the longest recovery time observed in the short-circuit test is 5 s. These results significantly outperform sensors with larger particle sizes in terms of measurement accuracy, stability, short-circuit recovery, and applicable temperature range. The findings demonstrate that the particle size of the reference electrode plays a crucial role in the measurement performance of LSCF/air oxygen sensors. Reducing the particle size of the reference electrode powder optimizes the material’s surface-to-volume ratio, thereby significantly enhancing oxygen ion transfer efficiency and catalytic activity during measurement. This study provides valuable insights for the development of low-temperature LBE oxygen sensors.