<p>The poor corrosion resistance and wear resistance of Mg alloys in harsh operating environments, coupled with the difficulty of existing protective coatings in simultaneously providing long-term corrosion barriers and excellent tribological properties, have become the core challenges limiting their engineering applications. This study investigated the effect of diamond particle size on the corrosion resistance and wear resistance of micro-arc oxidation/epoxy composite coatings. The experimental results demonstrate that the introduced diamond particles effectively filled the micropores and micro-cracks formed within the epoxy resin matrix during curing. This significantly improved the density and structural integrity of the coating, thereby enhancing its overall mechanical properties and wear resistance. Moreover, the uniform dispersion of diamond particles within the resin formed a dense physical barrier structure. This structure effectively blocked the penetration and diffusion of corrosive media, significantly improving the long-term corrosion resistance of the Mg alloy matrix under harsh environmental conditions. Therefore, the micro-arc oxidation/diamond epoxy composite coating not only exhibits outstanding corrosion and wear resistance but also offers advantages such as simplified preparation processes, controllable costs, and extended service life. This provides an economical, efficient, and durable technical solution for Mg alloy surface protection.</p>