A 600 W unit boiler plate type superheater leakage, resulting in the unit abnormal shutdown. The causes of superheater leaking were comprehensively determined through on-site inspection, macroscopic, microscopic, thickness measurement, chemical composition analysis, combined with metallographic structure analysis and hardness analysis. The results show that the transverse fatigue cracks occur on the inner arc surface of the outer wall of the tube and extend to the inner wall due to the unreasonable structure of the tube and the long period of alternating load. The initial leakage of high temperature and high pressure steam in this position leads to the chain blowing of other pipe sections, which ultimately leads to the occurrence of the unit shut down abnormally; The original defects, such as scratches and grooves, are the source of cracks in the neutral surface of the superheater elbow, which expand to the outer wall and form penetrating cracks under the influence of the process of burst and steam leakage. In order to prevent similar accidents from happening again, corresponding preventive measures are put forward in maintenance, supervision, design and manufacturing.