Abstract: This study explores the microstructural evolution and microhardness changes of CF8M stainless steel before and after cryogenic treatment at -196°C, providing theoretical and technical support for optimizing materials in high-pressure seals and low-temperature valves for high-tech and commercial ships. Metallographic analysis, microhardness testing, and nanoindentation were used to assess phase composition, microstructure, and mechanical properties. Results show that cryogenic treatment refines the ferrite morphology from blocky and network-like to lamellar or acicular, enhancing structural compactness and orientation. Partial residual austenite transforms into martensite, leading to increased local hardness and a more uniform distribution. Nanoindentation tests indicate reduced indentation depth post-treatment, reflecting improved plastic deformation resistance and higher elastic modulus. This study offers experimental and theoretical insights for enhancing CF8M stainless steel’s cryogenic performance, advancing material design for extreme environments.