The role of surface hydroxyls is significant for understanding catalytic performance of metallic oxides for CO2 electroreduction reaction (CO2ER). This Communication describes, employing SnOx as a model system, how to moderate coverage of hydroxyl to derive a stable Sn branches catalyst for CO2ER with a 93.1% Faradaic efficiency (FE) of carbonaceous products. With use of in situ attenuated total reflection surface enhanced infrared adsorption spectroscopy (ATR-SEIRAS) and density functional theory (DFT) calculations, we found that a proper amount of surface hydroxyls offered effective sites to boost CO2 adsorption via hydrogen bond. However, a higher surface coverage of hydroxyls leads to self-reduction of Sn-OH. We also explained the competition between self-reduction and CO2 reduction over Sn-based catalysts. The findings revealed the quantitative correlation between surface coverage of hydroxyl and CO2ER activity and suggested a logical extension to other metal oxide catalysts for CO2ER.