Abstract: The leakage of pressure pipeline in petrochemical plant can not work normally, the main influencing factors of cracking weld of pipe flange are traced, and the failure analysis of cracking weld of pipe flange is carried out. Through the observation and analysis of the leaking pipe flange specimen, the heat affected zone of the pipe flange weld cracks, and the cracking mode is brittle delayed fracture, and the mechanism is stress corrosion cracking.The stress corrosion is caused by the existence of quenched martensite in the weld heat affected zone, which leads to the hardness exceeding the standard seriously, and the stress corrosion cracking occurs under the action of wet hydrogen sulfide. The formation of hardened martensite should be related to the quality of welding heat treatment and testing.