Two-dimensional　(2D)　materials　have　shown　extraordinary　performances　as　photocatalysts　compared　to　their　bulk　counterparts.　Simulations　have　made　a　great　contribution　to　the　deep　understanding　and　design　of　novel　2D　photocatalysts.　Ab　initio　simulations　based　on　density　functional　theory　(DFT)　not　only　show　efficiency　and　reliability　in　new　structure　searching,　but　also　can　provide　a　reliable,　efficient,　and　economic　way　for　screening　the　photocatalytic　property　space.　In　this　review,　we　summarize　the　recent　developments　in　the　field　of　water　splitting　using　2D　materials　from　a　theoretical　perspective.　We　address　that　DFT-based　simulations　can　fast　screen　the　potential　spaces　of　photocatalytic　properties　with　the　accuracy　comparable　to　experiments,　by　investigating　the　effects　of　various　physical/chemical　perturbations.　This,　at　last,　will　lead　to　the　enhanced　photocatalytic　activities　of　2D　materials,　and　promote　the　development　of　photocatalysis.　©　The　Royal　Society　of　Chemistry　2017.