Abstract: The steam-side corrosion damage characterization and ultrasonic testing technique of the internal surface of water-cooled wall tube made of 20G steel removed from boiler of some power plant had been investigated by metallurgic inspection, scanning electron microscope-energy dispersive and other tests. The examination results showed that the reaction process of hydrogen and cementite in grain boundary caused decarburization of water-cooled wall tube, void fraction and grain boundary cracks initiation and propagation, and mechanical performance degradation. Due to the fact that the ultrasonic transverse wave is sensitive to the hydrogen corrosion cracks, small chip size of transverse wave probe of K1, 5MHz frequency is used to test the corrosion grain boundary cracks, showing high reliability. The wave form characteristics of the echo by hydrogen corrosion cracks in the samples and its equivalent amplitude can be used to assess the degree of hydrogen corrosion cracks.