Abstract: The 304 stainless steel thermocouple casing of a power plant cracked. The causes of casing cracking were analyzed by macroscopic observation, wall thickness measurement, chemical composition analysis, metallographic examination, hardness test, scanning electron microscope and energy spectrum analysis, immersion test and other methods. The results show that the surface quality of the casing was poor. Under the action of Cl^– and S^2–, the casing produced stress corrosion cracks. The condensed water in the air entered the casing along the micro-cracks, and the condensed water was absorbed by the insulating material ( mainly composed of MgO ), resulting in the volume expansion of the casing insulation layer and accelerating the stress corrosion cracking of the casing.