Designing　and　developing　the　highly　efficient　photocatalysts　is　full　of　significance　to　achieve　spontaneous　photolysis　water.　In　this　work,　using　the　first-principles　calculations,　we　have　performed　a　systematic　theoretical　study　of　water　splitting　photocatalytic　activity　of　the　InSe/g-CN　heterojunction.　It　is　concluded　that　the　InSe/g-CN　heterojunction　is　a　typical　type-II　semiconductor,　whose　electrons　and　holes　can　be　effectively　separated.　And　the　potential　of　the　conduction　band　minimum　(CBM)　and　valence　band　maximum　(VBM)　satisfy　the　requirements　for　photolysis　water.　Moreover,　the　changes　of　Gibbs　free　energy　(Delta　G)　of　the　oxygen　evolution　reaction　(OER)　and　the　hydrogen　evolution　reaction　(HER)　are　calculated　to　investigate　thermodynamic　sustainability　of　photolysis　water.　The　results　show　that　when　pH　=　7,　the　potential　driving　force　provided　by　the　InSe/g-CN　heterojunction　can　ensure　the　spontaneous　progress　of　HER　and　OER.　In　addition,　it　is　found　that　the　solar　conversion　efficiency　(eta(s))　of　the　InSe/g-CN　heterojunction　is　up　to　13.7%,　which　indicates　it　has　broad　commercial　application　prospects.　Hence,　the　InSe/g-CN　heterojunction　is　expected　to　be　an　excellent　candidate　for　photolysis　water.　(C)　2021　Published　by　Elsevier　B.V.　on　behalf　of　Chinese　Chemical　Society　and　Institute　of　Materia　Medica,　Chinese　Academy　of　Medical　Sciences.