Abstract: Dense transverse cracks were found on the backfire surface of the water-wall tube in the soot blower cover after the supercritical boiler ran for 1.2×10⁴ h. By data investigation, material composition inspection, and analysis of microstructure, fracture morphology and tensile properties, the causes of surface cracks of the water-wall tube were investigated. The results show that the material of the water-wall tube was normal, and the cracks on the tube outer wall of the backfire surface were thermal fatigue cracks. The outer diameter of the soot blower cover was too large, making the cover being topped on the water-wall tube around the soot blowing hole. Meanwhile, the temperature was low, making the soot blowing steam carrying water. The water dropped along the gap between the water-wall tube and the soot blowing sleeve to the backfire surface of the water-wall tube, causing the bearing of long-term heat and cold alternating stresses, and finally thermal fatigue cracking occurred.