Single-atom　catalysts　(SACs)　have　received　intensive　interest　due　to　the　utmost　atom　utilization　and　unique　catalytic　behaviors.　However,　the　catalytic　mechanism　of　SACs　is　still　unclear,　especially　in　the　reactions　involving　multiple　substrates.　Here,　we　report　that　the　environment　moieties,　protic　solvents,　and　external　bases　can　boost　the　cobalt　SAC-catalyzed　chemoselective　hydrogenation　of　nitroarenes.　Systematical　studies　clearly　reveal　that　the　heterolysis　of　H2between　the　metal　center　and　the　neighboring　coordination　sphere　is　significantly　facilitated　via　the　H-shuttling　or　deprotonation　route　with　the　participation　of　protic　solvents　or　bases.　Besides,　the　derived　single-atom　cobalt　hydride　species　are　confirmed　to　be　specifically　reactive　for　the　reduction　of　the　nitro　group,　affording　a　superior　activity　and　general　chemoselectivity　in　the　hydrogenation　of　various　nitroarenes.　Finally,　a　catalytic　cycle　is　proposed　to　elucidate　the　activity　origin　of　SACs,　which　sheds　light　on　the　rational　design　of　SACs　by　engineering　the　microenvironment　of　active　sites.　©　2022　American　Chemical　Society.　All　rights　reserved.