Abstract: The heat transfer tube of high temperature sampling cooler in a nuclear power station cracked. The causes of cracking on heat transfer tube of high temperature sampling cooler in nuclear power plant were analyzed by means of macro observation, chemical composition analysis, metallographic examination and hardness test. The results show that the cracking of heat transfer tube is alkali stress corrosion cracking caused by phosphate. The main reason was that there was local superheated leading to water boiling at the top of the heat transfer tube, resulting in phosphate concealment. The concentration and deposition of phosphate products, the enrichment and alkalization of local OH^- under the deposited products, the further dissolution of Fe₃O₄ oxide film, and even the direct reaction with fresh metal matrix, led to the formation of uneven corrosion pits on the surface, and eventually led to the leakage of heat transfer tube due to alkali induced stress corrosion cracking under the effect of residual stress. In addition, the higher hardness of heat transfer tube also promoted the crack initiation.