Developing　photocatalysts　with　active　sites　that　have　appropriate　interactions　with　both　N-2　and　reactive　intermediates　has　proved　to　be　feasible　for　direct　nitrogen　reduction　but　is　still　a　formidable　challenge.　Herein,　a　medium-spin　Au3Fe1/Mo　single-atom　alloy　photocatalyst　with　optical　antenna　structure　is　fabricated　through　an　alloying　strategy.　Fe　atoms　of　a　medium-spin　state　anchored　on　Au　nanoparticles　at　the　single-atom　level　via　Au-Fe　bonding　is　confirmed　by　combined　characterizations　of　aberration-corrected　high-angle　annular　dark　field　scanning　transmission　electron　microscopy　(AC-HAADF-STEM),　X-ray　absorption　fine　structure　(XAFS),　and　M　&　ouml;ssbauer　spectroscopic　techniques.　With　strong　Mo-Fe-Au　electronic　interactions,　the　Fe　sites　act　as　intrinsic　centers　apt　for　nitrogen　adsorption　and　activation,　which　is　conducive　to　the　preferential　cleavage　of　the　N　equivalent　to　N　bond　and　modulate　adsorption　of　reactive　intermediates.　Due　to　synergistic　effect　of　Au　nanoparticles　acting　as　optical　antennae,　the　Au3Fe1/Mo　photocatalyst　showed　excellent　photocatalytic　nitrogen　reduction　reaction　(pNRR)　performance,　giving　an　ammonia　formation　rate　of　484.2　mu　mol　h(-1)　g(-1)　and　solar-to-ammonia　(STA)　conversion　efficiency　up　to　0.12%.