To investigate the corrosion failure of a reducing tee at a shale gas station, a failure analysis was carried out. The failed tee was made of L245N steel, with a large-diameter branch of DN 80 and a small-diameter branch of DN 25. The DN 80 branch conveyed shale gas, while the DN 25 branch served as a vent and remained closed. The tee was after approximately 20 months of service. By means of macroscopic observation, transported medium analysis, chemical composition testing of the pipe fitting, hardness testing, metallographic structure examination, and analysis of the composition and morphology of corrosion products, the corrosion failure mechanism was identified. The chemical composition and mechanical properties of the tee met the standard requirements, the inclusion content was low, and the microstructure was normal. A substantial quantity of loose, lamellar and block-like corrosion products had formed at and around the perforation. The corrosion pit regions were mainly composed of Fe, C, O, and a small amount of S. The corroded areas exhibited pitting, ring-like corrosion, and mesa attack. The corrosion products were predominantly FeCO₃, with a small amount of FeS. Comprehensive analysis indicates that the synergistic corrosion effect of carbon dioxide (CO₂) and sulfate-reducing bacteria (SRB) is the root cause of the corrosion perforation of the tee.