Abstract: A fan coil exhaust valve cracked only used for 3 months,and the cracking reasons were analyzed by means of macro examination,fracture analysis,energy spectrum analysis and metallographic examination.The results show that the cracking of the exhaust valve originated from the inner wall.Because the inner wall touched the medium containing high content elements of Cl and S and there was large internal stress induced by the installation force,stress corrosion cracks formed under the co-action of above two factors.The cracks propagated continuously until penetrating the whole wall thickness and resulting in cracking.