Abstract: The welded seam on welded joint between flange and steel pipe of wet gas compressor of a platform cracked. The cracking causes of the welded seam were analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, mechanical property test, scanning electron microscopy analysis and energy spectrum analysis. The results show that the unreasonable structure of the welded joint and the incomplete fusion defects in the welded seam caused the local stress concentration, which led to the decrease of the fatigue limit of the welded joint. Under the action of stress, the crack source was formed at the interface between the weld root of the notch on one side of the flange and the incomplete fusion flange base metal. Under the action of the external cyclic load, the cracks gradually expanded to the outer surface. When the fatigue limit of the welded joint was reached, the welded seam cracking occured.