The charge-compensated N-F co-doping in combination with N or F mono-doping has been adopted to reduce the band-gap of KNbO so as to improve its photocatalytic efficiency under visible light for hydrogen generation by water splitting. The relative positions of N-F co-doping into O-sites of KNbO are considered to simulate the samples prepared by different experimental techniques. It is energetically unfavorable to form N mono-doped KNbO under any conditions, and the presence of F favors the N doping into KNbO. Although N doping reduces the effective band-gap, the localized states above the Fermi level and the presence of charge-compensated oxygen defect will accelerate the electron-hole recombination and thus reduce photocatalytic efficiency, while F doping hardly affects the band-gap. The defect introduced by N doping has been completely passivated in the N-F co-doped system. All the N-F co-doped configurations considered here have suitable band-gaps to absorb visible light with respect to the water redox level. Even the N-F co-doped system with lower dopant concentration harvests obviously longer wavelength of visible light as compared to the pure system.