A　series　of　Cu2O　clusters/MIL-100(Fe)　heterojunctions　with　different　Cu2O　mass　ratios　were　constructed　as　the　photocatalysts　for　N2　fixation.　An　optimal　nitrogen　fixation　performance　of　51.22　μmol·g−1·h−1　is　achieved　when　the　mass　fraction　of　Cu2O　is　20　%.　Isotope　analysis　experiments　confirmed　that　the　nitrogen　source　is　N2.　Possible　intermediate　signals　during　photocatalytic　reactions　were　detected　by　in　situ　DRIFTS.　XPS　results　further　confirm　the　formation　of　Cu2O/MIL-100(Fe)　heterojunctions,　and　that　the　existence　of　Cu–O　surface　interactions　is　the　key　to　efficient　electron　transfer.　It　is　revealed　that　the　improvement　of　the　catalytic　performance　is　derived　from　the　formation　of　heterojunction　which　regulates　the　energy　band　structure　as　well　as　promotes　the　separation　of　photogenerated　electron-hole　pairs.　This　study　proposes　a　possible　mechanism　of　photocatalytic　nitrogen　fixation　and　emphasizes　the　role　of　surface　interaction　and　electron　transfer　in　type-II　heterojunction　in　improving　nitrogen　fixation　performance.　©　2023　Elsevier　B.V.