Electroreduction of CO 2 in highly acidic environments holds promise for enhancing CO 2 utilization efficiency. Due to the HER interference and structural instability, however, challenges in improving the selectivity and stability toward multicarbon (C 2+ ) products remain. In this study, we proposed an "armor protection" strategy involving the deposition of ultrathin, hydrophobic SiO 2 onto the Cu surface (Cu/SiO 2 ) through a simple one-step hydrolysis. Our results confirmed the effective inhibition of HER by a hydrophobic SiO 2 layer, leading to a high Faradaic efficiency (FE) of up to 76.9% for C 2+ products at a current density of 900 mA cm -2 under a strongly acidic condition with a pH of 1. The observed high performance surpassed the reported performance for most previously studied Cu-based catalysts in acidic CO 2 RR systems. Furthermore, the ultrathin hydrophobic SiO 2 shell was demonstrated to effectively prevent the structural reconstruction of Cu and preserve the oxidation state of Cu δ+ active sites during CO 2 RR. Additionally, it hindered the accumulation of K + ions on the catalyst surface and diffusion of in situ generated OH - ions away from the electrode, thereby favoring C 2+ product generation. In situ Raman analyses coupled with DFT simulations further elucidated that the SiO 2 shell proficiently modulated *CO adsorption behavior on the Cu/SiO 2 catalyst by reducing *CO adsorption energy, facilitating the C-C coupling. This work offers a compelling strategy for rationally designing and exploiting highly stable and active Cu-based catalysts for CO 2 RR in highly acidic environments.