Abstract: In order to investigate the fracture causes and mechanisms of water-cooled wall tube in coal-fired boiler of a certain power plant, the failed water wall tubes were analyzed through macroscopic observation, mechanical property testing, material composition analysis, metallographic examination, scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS). The results showed that the chemical composition of the sample and the yield strength on both the fire-facing and back sides complied with the requirements stipulated in the relevant standards for 20G steel. However, the hardness, the tensile strength and the elongation after fracture had not reached the standard requirements. There was a thick oxide film on the inner wall of water wall tubes. Around the burst fracture, there were numerous of intergranular microcracks (fissures) propagating into the matrix interior, and severe decarburization occurred around the cracks. All these were obvious characteristics of hydrogen corrosion. At the fracture surface, there were corrosive elements such as sulfur and phosphorus, indicating that acid corrosion also existed. In addition, the weld seam was an area prone to acid corrosion, and the fusion line area was the weakest zone of the entire welding joint. As the corrosion-induced microcracks gradually propagated, fracture preferentially initiated at the fusion line region. Based on the above, it could be concluded that the fracture of the water wall tubes was caused by the combined effects of hydrogen corrosion and acid corrosion.