<p>Bamboo is a naturally-optimized composite material, but it is prone to deformation, cracking, mildew, and decay. The application of coatings with both mildew and decay resistance can provide long-term protection for bamboo, but bamboo lacks transverse cells and contains few fiber cell cavities and pits, leading to poor coating adhesion. This study subjected <i>Phyllostachys edulis</i> (<i>P. edulis</i>) and <i>Neosinocalamus affinis</i> (<i>N. affinis</i>) substrates to hygrothermal pretreatment to enhance the adhesion of coatings applied to them. A novel nano-Ag/SiO₂-modified water-based coating with excellent dispersibility and thermal stability was applied to the surface of the pretreated bamboo strip substrates. The adhesion of the coating reached the highest level (Grade 0, no coating detachment), and its initial and maximum peel forces increased by 53.8% and 41.1% after hygrothermal pretreatment, respectively. Upon increasing the nano-Ag/SiO₂ content, the thickness, adhesion, hardness, scratch resistance, resistance to heating-cooling cycles, and mildew/decay resistance of the water-based coating improved. After the mildew resistance tests, the infection level of the samples decreased from Grade 3 (1/2–3/4 of the surface) to Grade 0 (No visible hyphae), while the discoloration level decreased from Grade 4 (severe surface and internal discoloration) to Grade 1 (minor surface discoloration) after soil burial tests. The weight loss rate, gloss reduction, and color difference of the surface also decreased after both tests. The results showed that the water-based coating modified with 2 wt% nano-Ag/SiO₂ exhibited the best dispersion uniformity, abrasion resistance, and minimal color difference, providing the best decorative and protective performance for the bamboo. The coating extended the service life of bamboo and enhanced its applicability in high-humidity indoor environments and harsh outdoor conditions.</p>