In　this　work,　we　studied　the　electronic　band　structure　of　the　halogen　(F,　Cl,　and　Br)　functionalized　graphdiynes　(GDYs)　by　using　hybrid　density　functional　theory.　The　results　revealed　that　the　bandgap　energies　of　modified　GDYs　increase　as　the　number　of　halogen　atoms　increases.　It　is　also　found　that　the　position　of　the　valence　band　maximum　(VBM)　is　influenced　by　the　electronegativity　of　halogen　atoms.　The　higher　the　electronegativity,　the　deeper　the　VBM　of　the　GDYs　modified　by　the　same　number　of　halogen　atoms.　Importantly,　our　results　revealed　that　the　bandgap　of　GDY　could　be　effectively　tuned　by　mixing　types　of　halogen　atoms.　The　new　generated　conduction　band　and　valence　band　edges　are　properly　aligned　with　the　oxidation　and　reduction　potentials　of　water.　Further　thermodynamic　analysis　confirms　that　some　models　with　mixing　types　of　halogen　atoms　exhibit　higher　performance　of　overall　photocatalytic　water　splitting　than　non-mixing　models.　This　work　provides　useful　insights　for　designing　efficient　photocatalysts　that　can　be　used　for　overall　water　splitting.