In view of the fracture leakage accident of the circumferential welded joint of a styrene multiple heat exchanger in a petrochemical plant under service conditions, the cause of the accident was determined combined with theoretical discussion through crack morphology observation, simulation heat treatment test for small workpieces, microstructure observation, hardness test and other test methods, some improvement schemes were proposed, and a good result was obtained. The root cause of the leakage during operation of the equipment is that the existing local post-welding heat treatment process can not completely eliminate the welding residual stress of this thick plate structure. During the service of the equipment, the working stress is superimposed on the high circumferential residual stress, and the local stress exceeds the breaking strength of the material itself, resulting in transverse cracks, which cause penetration along the thickness direction and eventually lead to leakage accidents. The overall post-welding heat treatment process with better temperature field uniformity can eliminate residual stress, reduce joint hardness, enhance ductility, improve its cracking resistance under harsh working conditions, and avoid the occurrence of such leakage accidents.