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Based on first principles with the Tran-Blaha modified Becke-Johnson potential, pure and doped Ba5Nb4O15 have been investigated as highly efficient photocatalysts for solar water splitting. The results showed that the calculated band gap of Ba5Nb4O15 is improved to 3.81 eV by the new potential. Mono-doping with either anion (N, P) or cation (Mo) creates some partially occupied states at the band edges of Ba5Nb4O15. N–Mo co-doping and P–Mo co-doping can prevent the partially filled states from appearing at band edges and hence enhance the photocatalytic efficiency. The band edges of the co-doped systems still satisfy the requirements for spontaneous water splitting. More importantly, the band gap of P–Mo co-doped system is sharply reduced to 2.31 eV, being in ideal visible light region. The defect formation energy calculations show that P–Mo co-doped Ba5Nb4O15 is energetically favorable under O-rich and Nb-poor conditions. These show that P–Mo co-doped Ba5Nb4O15 has high efficiency for water splitting under visible light.