Abstract: The overheat pipe of water wall of a boiler cyclone separator cracked many times. The cause of cracking was analyzed by means of macroscopic observation, chemical composition analysis, metallographic examination, hardness test, scanning electron microscope and energy spectrum analysis. The results show that the super heat pipe was in a high temperature state above 450 ℃ for a long time, and the pearlite spheroidization was obvious. The deterioration of the microstructure led to the decrease of the strength and hardness of the super heat pipe. The heat pipe thinned by oxidation corrosion swelled and deformed under the action of steam pressure, which eventually led to the cracking of the heat pipe.