With the continuous enhancement of the strategic significance of special application fields such as deep sea，deep space，and polar regions，a long-term and stable energy supply has emerged as a crucial challenge. Underwater micro-nano nuclear energy systems，characterized by their high energy density，long service life，and immunity to environmental influences，have demonstrated immense application potential in these domains. A survey is conducted on the research status of underwater micro-nano nuclear energy systems in the United States，the former Soviet Union/Russia，and China，and the key technical challenges they confront are analyzed，encompassing nuclear fuel research and development，new material R&D，advanced processing technologies，and advanced instrumentation and control systems. Furthermore，the advanced coupling analysis technology of micro-nano heat pipe reactor nuclear energy systems developed based on the reactor Monte Carlo software RMC and the open-source CFD software OpenFOAM is introduced，and verification work is carried out based on the typical heat pipe reactor KRUSTY， validating the effectiveness and reliability of the coupling method. The future research and development of underwater micro-nano nuclear energy systems will provide significant technical support for domains such as deep sea exploration，resource exploitation，and national defense security.
