The　permeation　of　marine　water　can　lead　to　severe　degradation　of　the　protective　properties　of　polymer　composite　coatings.　At　present,　experimental　studies　on　the　anticorrosive　and　mechanical　properties　of　polymer　composite　coatings　are　relatively　mature,　but　the　impermeability　and　mechanical　property　changes　of　the　coating　during　the　permeation　process　are　challenging　tasks　to　be　studied　only　by　experiments.　In　this　paper,　molecular　dynamics　(MD)　models　of　composite　coatings　consisting　of　pure　epoxy　resin　(EP),　graphene/epoxy　resin　(Gr/EP),　and　graphene　oxide/epoxy　resin　(GO/EP)　were　constructed　respectively.　MD　simulations　were　performed　to　reveal　the　impermeability　and　mechanical　property　changes　of　composite　coatings　from　an　atomic　perspective.　The　results　show　that　GO　has　strong　interfacial　interaction　with　EP　and　the　best　effect　to　enhance　the　impermeability　and　mechanical　properties　of　epoxy　coatings.　NaCl　solutions　can　damage　the　coating　microstructure　and　occupy　the　Gr　and　GO　surfaces,　causing　them　to　no　longer　have　an　enhancing　effect.　The　study　of　GO/EP　coatings　with　different　permeation　degrees　shows　that　the　mechanical　properties　of　the　coating　decrease　as　the　penetration　degree　increases.