Low-mass　interfacial　contacts　and　poor　charge　transfer　efficiency　severely　limit　the　CO2　photoreduction　performance　of　semiconductor　heterojunctions.　In　this　work,　g-C3N4/Ag@Ag3PO4　S-scheme　heterojunction　is　prepared　by　an　in-situ　growth　method.　The　main　product　for　CO2　photoreduction　is　CO　with　the　selectivity　of　97　%.　The　evolution　rate　of　CO　for　optimal　g-C3N4/Ag@Ag3PO4　heterojunction　is　123.8　mu　mol　g-　1　h-1,　which　are　8.3and　3.4-fold　higher　than　those　of　pristine　Ag3PO4　and　g-C3N4,　respectively.　It　is　found　that　Ag3PO4,　Ag,　and　gC3N4　form　strong　interacting　interfacial　structure,　in　which　Ag3PO4　links　with　Ag　nanoparticles　through　tandem　Ohmic　contact.　More　importantly,　sigma　bonds　are　formed　via　hybridization　of　px　orbital　for　C　and　N　with　dx2-y2　orbitals　of　Ag　in　g-C3N4/Ag@Ag3PO4　S-scheme　heterojunction,　establishing　an　atomic-level　dx2-y2(Ag)-px(C,N)　charge　flow　highway.　This　work　provides　new　viewpoints　into　the　design　of　heterojunction　photocatalysts　with　high-quality　interfaces　and　highlights　the　insights　of　charge　transfer　behavior　in　regulating　the　catalytic　activity.