Abstract: The cracking reason of a large diameter flange was analyzed by means of macroscopic observation, chemical composition analysis, hardness test, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the matrix structure of the flange heat affected zone was sensitized, and the overlap between the cross section changed and the heat affected zone led to the superposition stress. The grains of the flange matrix structure were coarse and contained a large number of strip inclusions. The flange ran within the sensitized temperature range for a long time, which eventually led to stress corrosion cracking of the flange. Finally, according to the specific cracking reasons, the corresponding improvement measures were put forward.