Ultrathin　two　dimensional　(2D)　materials　have　triggered　extensive　interest　for　their　exceptional　properties　and　potential　applications.　Herein,　atomic　layer　graphitic　carbon　nitride　(g-C3N4)　was　obtained　by　a　simple　ultrasonic　exfoliation　approach,　and　cadmium　sulfide　(CdS)　nanoparticles　were　successfully　grown　on　these　ultrathin　g-C3N4　nanosheets　(UCNNS)　via　a　facile　solvothermal　method.　The　as-prepared　UCNNS　CdS　nanocomposite　exhibits　significantly　enhanced　photocatalytic　activity　for　methyl　orange　(MO)　degradation　under　visible　light　irradiation.　The　enhancement　of　the　photocatalytic　activity　should　be　attributed　to　the　well-matched　band　structure　and　intimate　contact　interfaces　between　the　UCNNS　and　CdS,　which　lead　to　the　effective　transfer　and　separation　of　the　photogenerated　charge　carriers.　The　mechanism　for　the　photodegradation　of　MO　by　the　composite　was　also　investigated　in　this　study.　This　study　highlights　the　potential　applications　of　atomic　layer　g-C3N4　based　photocatalysts,　and　we　hope　our　work　may　provide　a　new　insight　for　the　construction　of　photocatalysts　with　efficient　visible　light　activity.