Many　highly　active　electrocatalysts　for　the　reduction　of　N-2　to　NH3　(NRR)　have　been　synthesized　but　suffer　from　poor　selectivity.　One　crucial　reason　is　that　the　adsorption　of　hydrogen　often　dominates　at　the　active　centers　at　applied　voltage,　which　leads　to　the　competitive　hydrogen　evolution　reaction.　This　work　used　density　functional　theory　(DFT)　calculations　to　develop　a　class　of　stable　polyoxometalate-based　electrocatalysts　including　phosphomolybdic-,　phosphotungstic-,　silicotungstic-,　and　silicomolybdic-acid　supported　Ru　single　atoms　to　efficiently　catalyze　the　NRR　process　with　an　overpotential　lower　than　0.25　V.　More　importantly,　phosphomolybdic-　and　phosphotungstic　acid-supported　Ru　electrocatalysts　can　achieve　high　selectivity　at　applied　voltage.　This　work　offers　useful　insights　into　designing　high-performance　polyoxometalate-based　electrocatalysts　for　the　NRR.