The　hybrid　PBE0　density　function　has　been　employed　to　investigate　the　mechanisms　of　the　photocatalytic　performance　of　ZnIn2S4.　By　calculating　the　electrostatic　potential,　energy　band　structure,　density　of　states,　effective　mass　of　photogenerate　charge　carriers　and　visible-light　response,　we　confirm　that　ZnIn2S4　is　an　excellent　photocatalyst　for　water　splitting　as　well　as　for　decomposing　organic　contaminants.　With　a　unique　layer　structure,　ZnIn2S4　has　an　internal　electronic　field　reducing　the　recombination　of　charge-hole　pairs.　The　factors　of　a　small　effective　mass,　a　large　m(h)*/m(e)*　ratio,　and　a　dispersive　conduction　band　figure　are　in　favor　of　the　high　migration　rate　and　the　efficient　separation　of　charge　carriers.　The　narrowed　direct　band　gap　with　the　incorporation　of　In　5s　and　In　5p　states　in　energy　bands　can　account　for　the　response　in　the　visible　light　region.