Abstract: This article conducts a failure analysis on the leakage issue occurring at the welded joint of a duplex stainless steel tee joint in the wellhead pipeline of an offshore gas field. The failure causes are comprehensively analyzed from aspects such as macroscopic and microscopic fracture morphologies, crack propagation paths, microstructural characteristics, phase proportions, and hardness of the welded joint. The research results indicate that cracks initiated at the weld toe of the pipeline's inner wall weld joint, penetrated through the heat-affected zone, and propagated towards the base material, leading to pipeline leakage. The main failure cause is stress concentration due to poor weld formation in the pipeline's inner wall weld joint, coupled with a corrosive environment, which induces stress corrosion cracking. A large amount of secondary austenite forms in the weld metal microstructure, leading to an increased austenite content and a ferrite content of only 20%, thereby reducing the resistance of the weld metal to stress corrosion cracking. The microstructure of base material exhibits a significant amount of blocky intergranular σ-phase intermetallic compounds, which make the base material brittle, accelerate crack propagation, and increase the risk of localized corrosion leakage.