The　interfacial　structure　plays　a　key　role　for　photoinduced　charge　transfer　and　molecular　activation　in　heterogenous　photocatalysis.　However,　the　specific　interfacial　electronic　structure　of　heterostructure　and　its　effect　on　the　photocatalysis　reaction　have　remained　unclear.　Herein,　insulator-based　heterostructure　(CaSO4-BiOI)　was　constructed　to　investigate　the　atomic　interfacial　structure.　The　interface　in　CaSO4-BiOI　heterostructure　presents　electron-rich　environment　which　could　enhance　the　electronic　interaction　between　catalyst　surface　and　reactants,　and　thus　promotes　activation　and　dissociation　of　reactants.　Taking　photocatalytic　NO　oxidation　as　target　reaction,　the　adsorption-reaction　process　was　dynamically　monitored　by　in　situ　DRIFT　and　plenty　of　reaction　intermediates　were　observed.　The　generation　of　new　activated　molecules　and　intermediates　could　provide　an　alternative　pathway　to　enhance　the　conversion　of　inert　pollutant.　Hence,　the　reactants　and　intermediates　can　be　smoothly　converted　to　the　final　products　with　the　assistance　of　electron-rich　environment　at　the　interface　of　heterostructure,　achieving　the　safe　and　efficient　photocatalytic　performance.