Abstract: Research work is performed on the failure of thin-walled gears used in mechanical transmission mechanism of air-lock in nuclear power plants, based on the failure mechanism of thin-walled gears and its loading condition. The essential causes of failure of thin-walled gears founded, and they are: a. Material selection of thin-walled gears is unreasonable. Raw material is ANSI 1340, and composition analysis results show that S element (content is 0.11%) is far beyond the standard requirement. The gears contain a large amount of MnS inclusions, which deteriorates the material properties, and resulting in tooth undercut fracture, b. Structure design of thin-walled gears is unreasonable. A special structural displacement gear with a number of 12 teeth is adopted, and the displacement of the gear exceeded its permissible limit, resulting in the fracture of the tooth. Finally, the optimization and improvement measures for thin-walled gears are proposed from aspects such as the material selection, the manufacturing process control and the structural design.