Abstract: Through chemical composition analyzing and mechanical property testing, the results are showed that the water wall tube is accordance with standard. Structure analyzing shows fin breadth is wider 2 times than that of other parts which makes water wall temperature higher than that of other position. Microstructure analyzing shows slight pearlite globalization which makes granular interface weaker. Water wall temperature fluctuates. Changeable thermal load will makes water wall tube expansion and contraction which makes water wall withstand thermal fatigue stress. Because holding parts which supports water wall tube restrain free expansion and contraction, thus stress increases resulting thermal fatigue cracking on weaker position. After crack forms, it continually expands under stress. Inner pressure assists cracking to expand. When cracking expands to the critical water wall tube thickness, tube thickness can not sustain inner pressure so as to explode instantly.