The electronic structure of Eu-doped NaTaO_3 in Na-rich environment is investigated by the first-principles theory. By simulating the two different models of Eu~(3+) ions selectively located in Ta and Na sites, respectively, the band gaps of two Eu-doped NaTaO_3 models were all narrowed, which were assigned to lattice defects and impurity band of the Eu dopent. For the model of Eu~(3+) ions located in the Na+ sites of NaTaO_3, the new impurity band mainly composited of Eu 4f orbital appeared at the top over the valence band, indicating the enhanced oxidative ability. For the model of Eu~(3+) ions located in the Ta5+ sites of NaTaO_3, a midgap state generated was located at the bottom of conduct band and the band potential shifted up, confirming the strong reductive ability in the Na-rich enviornment. The densities of electron states were significantly increased in both the conduction and valence bands in Na-rich model, which resulted in the increased carrier migration rate and thus photocatalytic activity enhancement. It is proposed that Eu~(3+) ions doping at the Ta sites could enhance the reduced photocatalytic performance via controlling the nonstoichiometric Na/Ta molar ratio in the Eu-doped NaTaO_3 system. The electronic structure of Eu-doped NaTaO_3 in Na-rich environment is investigated by the first-principles theory. By simulating the two different models of Eu ~ (3+) ions each located selectively in Ta and Na sites, respectively, the band gaps of For the model of Eu ~ (3+) ions located in the Na + sites of NaTaO_3, the new impurity band mainly composited of Eu 4f orbital was the top of the valence band, indicating the enhanced oxidative ability. For the model of Eu ~ (3+) ions located in the Ta5 + sites of NaTaO_3, a midgap state generated was located at the bottom of the conduct band and the band potential shifted up, confirming the strong reductive ability in the Na-rich enviornment. The densities of electron states were significantly increased in both the conduction and valence bands in Na-rich model, which resulted in the increased carrier migration rate and thus pho tocatalytic activity enhancement. It is proposed that Eu ~ (3+) ions doping at the Ta sites could enhance the reduced photocatalytic performance via the nonstoichiometric Na / Ta molar ratio in the Eu-doped NaTaO 3 system.