Abstract: During CO₂ leakage, the Joule-Thomson effect and operational factors such as throttling and venting can cause the local temperature of the pipeline to drop abruptly to -78.5 ℃, significantly increasing the risk of brittle fracture. This study aims to precisely determine the ductile-to-brittle transition temperature of CO₂ pipeline steel and elucidate how low temperatures affect its fracture behavior, providing theoretical guidance for crack arrest design and safety assessment of CO₂ pipelines. X52 line pipe steel was investigated through Charpy impact tests at various low temperatures and fracture surface morphology analyses to evaluate temperature effects on impact toughness. The temperature responses during crack initiation, stable propagation, and unstable propagation were examined to reveal the evolution of fracture behavior and mechanisms under low-temperature conditions. Results indicated that as temperature decreased, the material's impact energy absorption declined sharply, with fracture mode transitioning from ductile tearing to a mixed ductile-brittle mode, and finally to brittle cleavage. The ductile-to-brittle transition temperature ranged from -70 ℃ to -80 ℃, with -60 ℃ identified as the critical threshold where crack arrest capability deteriorated markedly.