Abstract: The maintenance work of high tritium heavy water systems is widely carried out in heavy water reactors. Most of these maintenance work not only poses risks of gamma external radiation and radioactive contamination, but also risks of tritium internal radiation that may exceed the occupational exposure dose limit. Compared with the relatively slow changes in external exposure, the rapid changes in tritium levels in the air caused by high tritium heavy water, coupled with many factors that affect the tritium internal exposure dose of workers, make it necessary to establish a complete radiation control process to address this risk. The authors take the replacement of the O-ring of the moderator main pump in CANDU-6 heavy water reactor as an example, and summarize the radiation protection methods in the maintenance work of high tritium heavy water systems from the aspects of work control, radiation risk analysis, radiation protection plan formulation, on-site implementation control, and implementation effect analysis, in order to provide some reference and guidance for related maintenance work of similar nuclear facilities.