Friction and wear behaviors severely inhibit the regular operation and lifetime of mechanical components served in the ocean extreme environment. Herein, we report a novel in-situ synthesis strategy for fabricating silicon dioxide nanoparticles-reinforced epoxy (EP)-based composite coatings. The crystalline phases, sizes, morphologies, and reaction mechanism of the in-situ-synthesized nanoparticles were characterized, and the mechanical and tribological properties of the EP-silicon dioxide based composite coatings were also investigated in detail. The results showed that the in-situ-synthesized nanoparticles with a size about 100-150 nm were in an amorphous state and were covalently grafted onto the epoxy resin matrix. The thermal stability, nano-hardness and scratch-resistance of the composite coatings have been significantly improved. Moreover, the appropriate nanoparticles greatly reduced the wear rates of the epoxy composite coatings, which decreased by two orders of magnitude than that of pure EP coating when their incorporation content reaches 20 wt%.Additionally, the wear rate of the 15 wt% composite coating under the seawater medium decreased by 79.5 compared to that of pure coating.