Three　dimensions　(3D)　flower-shaped　g-C3N4/MoS2　composite　was　prepared　by　immobilizing　molybdenum　di-sulfide　(MoS2)　on　carbon　nitride　(C3N4)　with　structural　defects　by　in-situ　hydrothermal　assay.　The　morphological　structure　and　photochemical　properties　of　the　obtained　catalysts　were　characterized　by　SEM,　TEM,　EDS,　FTIR,　XRD　and　XPS.　Using　methylene　blue　(MB)　as　the　target　pollutant,　the　photocatalytic　performance　cyclic　stability　of　g-C3N4/MoS2　composite　were　studied　under　simulated　sunlight.　The　results　showed　that　the　prepared　catalyst　could　remove　98　%　of　MB　within　60　min　and　still　reached　96.4　%　after　5　cycles　with　good　stability　and　regeneration.　Its　apparent　rate　constant　was　0.0523　min-1,　which　was　11.5,　8.2　and　13.9　times　higher　than　those　of　g-C3N4　(0.00453　min-1),　C3N4　with　structural　defects　(0.00635　min-1)　and　MoS2　(0.00377　min-1),　indicating　that　the　synergistic　effect　of　structural　defects,　heterojunctions　and　3D　structure　has　achieved　efficient　degradation　of　MB.　Furthermore,　the　photocatalytic　mechanism　of　g-C3N4/MoS2　composite　was　proposed　by　combining　free　radical　capture　experiment　and　electron　paramagnetic　resonance　(EPR)　spectroscopy　and　the　results　showed　that　that　center　dot　OH　and　center　dot　O2-　played　a　primary　role　in　the　catalytic　degradation.