The　insufficient　oxidation　capacity,　high　carrier　recombination　rate　and　limited　sunlight　absorption　seriously　suppress　the　photocatalytic　activity　of　pure　g-C3N4.　Using　state-of-the-art　hybrid　density　functional　theory,　we　report　an　efficient　method　to　tackle　all　aforementioned　issues　of　g-C3N4　by　metal-nonmetal　(S　and　K)　co-doping　here.　We　find　the　adsorption　of　K　atom　on　hollow　site　causes　dynamic　strain　of　g-C3N4.　The　S　+　K　co-doping　not　only　shifts　the　band　edges　downwards　to　achieve　a　much　large　overpotential　of　ca.　0.76　V,　but　also　significantly　extends　the　visible-light　absorption　threshold　of　g-C3N4.　More　importantly,　the　newly　established　channel　between　neighboring　heptazine　units　in　the　doped　structure　is　highly　favorable　for　the　separation　of　charge　carriers.　Our　results　help　the　design　of　high-performance　visible-light-responsive　g–C3N4–based　photocatalyst　for　solar　water　splitting.　©　2019　Hydrogen　Energy　Publications　LLC