Abstract: The welding joint of engine thermolator close to SUS430 shell cracked in testing progress. To find the mode and cause of the cracking, chemical composition analysis of thermolator shell, SEM fracture and microstructure observation of the welding joint are carried out. The service condition of thermolator before failure was also considered. The results indicate that intergranular corrosion led to the intergranular cracking of the welding joint close to the thermolator shell. And the corrosion cracking of shell at heat affected fine grain zone was mainly due to the intergranular corrosion sensitivity of the heat affected fine grain zone in welding joint and the corrosion environment from tap water in testing. Meanwhile, some reasonable improvements have been done for the cracking of welding joint in the thermolator and the effect was favorable.