Abstract: Lead-bismuth eutectic (LBE) has been demonstrated to exhibit excellent physical and chemical properties, positioning it as a leading candidate for the coolant of the fourth generation of preferred lead-bismuth reactors and accelerator-driven sub-critical system (ADS). However, lead-bismuth alloys are highly corrosive and are prone to generating impurities during operation. Without proper treatment, the insoluble impurities produced by LBE may compromise equipment functionality. Therefore, the removal of insoluble impurities in LBE is crucial. The objective of this study was to investigate the sources of insoluble impurities in the lead-bismuth operating circuit under low-temperature high content of oxygen conditions and to provide data to the purification of impurities in lead-bismuth reactors such as the Accelerator-Driven Transmutation Research Unit (CiADS). The objective of this study was to achieve the aforementioned goal. To that end, lead-bismuth flow tests were conducted. A lead-bismuth test stand was utilized to complete the experiment, and the 1 000 h lead-bismuth loop operation experiment was performed at the temperature of 385 ℃ in this study. The pressure of cover gas was 0.05 MPa and the material of loop was 316L stainless steel, which were selected as necessary to match the CiADS operating conditions and were intended to replicate the operating conditions in engineering practice. The impurities on the surface of the lead-bismuth alloy and the inner wall of the loop were obtained experimentally. A comprehensive array of analytical techniques, including SEM, EDS, and chemical analyses of impurities, was employed to systematically investigate impurity-related information from both qualitative and quantitative perspectives. This approach enabled a thorough examination of the elements identified during the research, facilitating a comprehensive and nuanced understanding of the impurity profile. The analysis results show that under low-temperature and high content of oxygen conditions, the insoluble impurities in the lead-bismuth alloy mainly consist of a mixture of lead and bismuth oxides, lead-bismuth residues, and trace amounts of iron oxide. These impurities take the form of irregular crystals, with dimensions ranging from 1 mm×1 mm×1 mm-3 mm×1 mm×1 mm. It was also found that the primary source of insoluble impurities in the circuit is contributed by the reaction between excess oxygen and lead-bismuth at operating temperature which results in the formation of oxides. These oxides gradually accumulate, with some floating on the surface of the lead-bismuth alloy and others depositing on the surface of the container inner wall. The results obtained in this study can be used for the design of the purification system in CiADS, and it will be also useful for the safety operation of the lead-bismuth cooled reactor.