Abstract: In order to evaluate the degree of hydrogen corrosion on the pressure equipment at high temperature and high hydrogen pressure, the possibility of diagnosing hydrogen corrosion at early stage by ultrasonic method has been studied. Specimens made of 20 steel and Q345R steel were exposed to hydrogen gas at 450 ℃ with a hydrogen partial pressure 18 MPa for 18 hours，and then its corrosion situations were measured by the velocity ratio, thickness measurement and TOFD methods. The results suggested that it was not easy to detect the hydrogen corrosion by the velocity ratio in this situation if the specimens were corroded lightly, but thickness measurement and TOFD could effectively diagnose the hydrogen corrosion at early stage.