Indium-based materials (e.g., In 2 O 3 ) are a class of promising non-noble metal-based catalysts for electroreduction of carbon dioxide (CO 2 ). However, competitive hydrogen reduction reaction (HER) on indium-based catalysts hampers CO 2 reduction reaction (CO 2 RR) process. We herein tune the interfacial microenvironment of In 2 O 3 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 In 2 O 3 themselves. The surface functionalization of alkyl phosphoric acids over In 2 O 3 is demonstrated to remarkably boost CO 2 conversion. For example, octadecylphosphonic acid modified In 2 O 3 exhibits Faraday efficiency for H 2 ( FE 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 In 2 O 3 counterparts ( FE 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 CO 2 molecules are trapped, increasing local CO 2 concentration at In 2 O 3 active sites, thus leading to the significantly enhanced CO 2 RR 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.