Indium-based materials (e.g., In₂O₃) are a class of promising non-noble metal-based catalysts for electroreduction of carbon dioxide (CO₂). However, competitive hydrogen reduction reaction (HER) on indium-based catalysts hampers CO₂ reduction reaction (CO₂RR) process. We herein tune the interfacial microenvironment of In₂O₃ through chemical graft of alkyl phosphoric acid molecules using a facile solution-processed strategy for the first time, which is distinguished from other researches that tailor intrinsic activity of In₂O₃ themselves. The surface functionalization of alkyl phosphoric acids over In₂O₃ is demonstrated to remarkably boost CO₂ conversion. For example, octadecylphosphonic acid modified In₂O₃ exhibits Faraday efficiency for H₂ (FE ${}_{{\mathrm{H}}_2}$) of as low as 6.6% and FE_HCOOH of 86.5% at −0.67 V vs. RHE, which are far superior to parent In₂O₃ counterparts (FE ${}_{{\mathrm{H}}_2}$ of 24.0% and FE_HCOOH of 63.1%). Moreover, the enhancing effect of alkyl phosphoric acid functionalization is found to be closely related to the length of alkyl chains. By virtue of comprehensive experimental characterizations and molecular dynamics simulations, it is revealed that the modification of alkyl phosphoric acids significantly alters the interface microenvironment of the electrocatalyst, which changes the electrocatalyst surface from hydrophilic and aerophobic to hydrophobic and aerophilic. In this case, the water molecules are pushed away and more CO₂ molecules are trapped, increasing local CO₂ concentration at In₂O₃ active sites, thus leading to the significantly enhanced CO₂RR and suppressed HER. This work highlights the importance of regulating the interfacial microenvironment of inorganic catalysts by molecular surface functionalization as a means for promoting the electrochemical performance in electrosynthesis and beyond.