Abstract: When the reactor of a sulfur recovery unit was only used for 2 years, its coil tube cracked. The causes of coil tube cracking were analyzed by means of macro analysis, chemical composition analysis, metallographic examination, hardness test, fracture analysis and scale analysis. The results show that the material of reactor coil tube was sensitized. Alkali embrittlement cracking occurred on the inner wall of the coil tube, and stress corrosion of polythionic acid cracking occurred on the outer wall of the coil tube. Under the joint action of the three, the coil tube cracked.