Abstract: A leakage failure occurred in the welded joint of the heat exchanger tube and tube plate in a high-pressure air cooler turbine. The leakage and critical positions are investigated by means of macroscopic observation, mechanical fracture and crack surface analysis, metallographic analysis, microhardness test, scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS). It is found out that the cracks at the failure location expands from the welded joint of the tube box to the surface of the expansion gap, and that a large amount of S element is present at the crack surface. The failure mode is determined to be hydrogen sulfide stress corrosion cracking (SSCC) by combining stress, fracture morphology, and elemental analyse. Due to the hydrogen sulfide in the tube, and the synergistic effect of the stress concentration and structural stress by the external load, SSCC cracking occurs in the weld toe, and the cracks propagates along the fusion line to the outer wall of the expanded tube, leading to the leakage failure.