Oxidation protection of active materials in liquid environments is essential for a number of environmental and energy related applications, including photocatalysis, water splitting, and wet electronics. While surface coating has been widely used, anti-oxidation self-protection of active materials is of both fundamental and technological interest but remains largely unexplored. Here, we report a study on localized surface plasmon resonance (LSPR)-mediated oxidation protection in copper-cuprous oxide core-shell microfibers. The microfibers are synthesized via a newly developed reagent-free electrolytic method and have unique interfacial structures and high surface activity. We find that this highly active composite material, which is normally unstable under ambient wet condition, is surprisingly stable in water upon its exposure to simulated sunlight irradiation. The LSPR-mediated self-protection mechanisms are supported by the experimental evidence and discussed. Our approach could be applicable to both preparation and studying LSPR effects of a wide range of metal based plasmonic materials.