Porous materials can serve as optimal supporters for the fabrication of confined plasmon nano-photocatalysts with high dispersity. The low-loading amounts of the confined nanoparticles (NPs) due to their easy-to-migrate tendency out of the pores, however, cause a bottleneck for the photocatalytic performance. We herein reported the in-situ growth of Ag NPs within thio-functionalized UiO-66 metal-organic frameworks (MOFs). Owing to the anchoring effects of the thiol groups, Ag nanoparticles were stabilized in the channels at ultrahigh loading amounts (up to 51.2%) for significantly enhanced plasmonic resonance. Through optimizing the loading amounts of confined Ag and the remaining pore volumes for mass diffusion, we achieved an exceptional catalytic activity for the photocatalytic reduction of CO₂ with Ag@MOFs. The photo-induced electron transfer rate is as high as 142.4 µmol·g^–1·h^–1, which is ~ 17.4 times higher than bare UiO-66-(SH)₂. Notably, the enhanced charge transfer kinetics, facilitated by the plasmon-induced hot-electron injections, enables the multiple-electron reduction of CO₂ to hydrocarbons. This work presents a straightforward strategy for constructing confined plasmon NPs with ultrahigh loading amounts, and demonstrates their remarkable performance in photocatalytic CO₂ reduction.