Abstract: Since the operation of unit 2 in a power plant, weld seam cracking and leakage of main steam pipeline have occurred twice. Taking the cracked straight pipe and elbow of unit 2 as samples, the hardness and wall thickness were tested, the chemical composition analysis, metallographic examination, scanning electron microscope analysis and energy spectrum analysis were carried out. The results show that the cracking failure mode of weld seam of the unit 2 main steam pipe is stress corrosion. Under the combined action of stress and corrosion environment, multiple initial cracks are cultivated and generated at the positions with great grain size difference between the normalizing fine-grain area and the tempering zone at the elbow side HAZ in the inner wall of the pipe. After crack initiation, it extends along the boundary between normalizing fine grain zone and tempering zone at the elbow side of weld, finally, the crack penetrates the pipe wall and the weld cracking leakage occurs.