Improving　catalytic　performance　by　controlling　the　microstructure　of　materials　has　become　a　hot　topic　in　the　field　of　photocatalysis,　such　as　the　surface　defect　site,　multistage　layered　morphology,　and　exposed　crystal　surface.　Due　to　the　differences　in　the　metal　atomic　radius　(Mn　and　Cd)　and　solubility　product　constant　(MnS　and　CdS),　Mn　defect　easily　occurred　in　the　S/Mn0.4Cd0.6S　(S/0.4MCS)　composite.　To　optimize　the　photocatalytic　performance　in　N2　fixation,　the　effects　of　the　synthesis　conditions　and　reaction　conditions　for　S/0.4MCS　were　explored　and　systematically　studied.　Combined　with　the　experimental　characterization　and　theoretical　calculation,　not　only　the　photocatalytic　reaction　pathway　but　also　the　key　steps　of　N2　reduction　were　explored.　Moreover,　the　transfer　mechanism　of　photogenerated　charge　carriers　(PCCs)　formed　between　S　and　0.4MCS　was　studied,　which　enhanced　the　utilization　rate　of　photogenerated　electrons　(e-)　and　holes　(h+).　This　work　detailedly　discusses　the　relationship　between　microstructure　and　photocatalytic　performance,　which　is　beneficial　for　the　design　of　efficient　photocatalyst.