Abstract: A failure accident of water screen tubes of boiler had been analyzed. Visual examination,mechanical test,metallographic examination,electron scanning microscope examination,X-ray diffraction examination had been used for this analysis. Lots of fine-cracks had been found on the inside wall of water screen tubes towards the fume. Deeply decrease of tensile strength and yield strength had been found when the water screen tubes towards the fume. The PH value of this boiler’s water system was out of control as acidic property,which was caused by a mistake of operation in the past time. Hydrogen brittle fracture had been confirmed as the reason of the fracture of water screen tubes.