Abstract: A check valve for the high－temperature and high－pressure water system of a certain power plant leaked, and upon inspection, it was found that the valve disc welding layer had cracked. The causes of valve disc cracking were analyzed using methods such as macroscopic observation, chemical composition analysis, metallographic examination, hardness testing, residual stress testing, scanning electron microscopy and energy spectrum analysis. The results show that the chemical composition of the valve disc welding layer was unqualified, and there were defects such as looseness and porosity. When the valve disc was in service, it was subjected to medium impact, and the external stress applied form stress concentration at the defect area, which superimposed with the residual stress of the valve disc welding layer. Cracks initiated and rapidly expanded at the defect area of the welding layer, ultimately led to the cracking of the valve disc.