Single-atom　catalysts　(SACs)　have　recently　emerged　as　stars　in　boosting　the　synthesis　of　NH3　from　N2,　as　the　catalytic　performance　of　the　supported　single　atoms　can　be　modulated　by　their　coordination　environment.　In　this　work,　we　propose　a　new　strategy,　based　on　comprehensive　density　functional　theory　calculations,　whereby　the　coordination　environment　of　a　single　Mo　atom　can　be　tuned　by　a　central　heteroatom　(X　=　Fe,　Co,　Ni,　Cu,　Zn,　Ga,　Ge,　and　As)　in　the　Kegging-type　polyoxometalate　(POM,　(XW12O40)n−)　substrate　to　catalyze　the　electrochemical　nitrogen　reduction　reactions　(NRR).　Firstly,　we　demonstrate　that　the　single　Mo　atom　binds　strongly　to　the　POM　surface　oxygen　hollow　sites　without　aggregation.　Secondly,　the　adsorption　of　⋆N2　on　the　POM-supported　Mo　atom　is　investigated　and　the　reactivity　is　assessed　by　calculating　the　thermodynamics　of　the　NRR.　The　results　show　that　the　POM　(X　=　Co　and　As)　supported　Mo　atom　has　high　NRR　activity　with　low　limiting　potentials.　Finally,　we　reveal　the　origin　of　the　NRR　activity　by　analyzing　the　electronic　structure.　The　results　show　that　the　charge　on　the　O　atoms　of　oxygen　hollow　sites　is　affected　by　the　central　heteroatom.　Due　to　such　effect,　it　can　be　found　that　more　d　electrons　are　transferred　from　Mo　supported　by　POM　(X　=　Co　and　As)　to　*N2,　thus　the　N≡N　triple　bond　is　activated.　This　strategy　of　coordination　environment　tuning　proposed　in　this　work　provides　a　useful　guide　for　the　design　of　efficient　catalysts　for　electrocatalysis.　[Figure　not　available:　see　fulltext.]　©　2022,　Tsinghua　University　Press.