The Sr₂FeTaO₆/NaTaO₃ heterojunctions with the controllable molar ratio and space of heterojunction transitional zone were fabricated using in situ hydrothermal method by adjusting hydrothermal temperature and time. Comparing with the pure NaTaO₃ nanosheets and Sr₂FeTaO₆ nanoparticles, the Sr₂FeTaO₆/ NaTaO₃ heterojunctions exhibit higher photocatalytic performances, due to introducing narrow-bandgap Sr₂FeTaO₆ component, which could obviously enhance its absorbability in visible light region and separation efficiency of photo-generated electron-hole pairs. Under visible light irradiation, the optimal Sr₂FeTaO₆/NaTaO₃ heterojunction, with the molar ratio of 0.53 and space of heterojunction transitional zone of 0.35 nm, shows the best photocatalytic activity with the NO removal ratio of 72% in 40 min, hydrogen evolution rate of 1334 μmol h⁻¹ g⁻¹ in 5 h, and apparent quantum efficiency (AQE) for hydrogen evolution of 38.8% at 420 nm. More interestingly, the space of heterojunction transitional zone, related to the interface bonding degree of heterojunction material, may not be the only factor which influences transferring and separating efficiency of photo-generated charge carriers, but it is certainly an important one, which provides a new cognitive perspective for constructing and understanding efficient heterostructure photocatalyst.