As a kind of the promising photocatalyst, double pervoskite oxides have the excellent light adsorption performance, highly stability, and favorable electronic property, but fabricating nanoscale double perovskite with high crystallinity and purity under facile preparation conditions is an attractive challenge. In this work, the low-temperature solvothermal method was introduced to fabricating double perovskite oxide SrFe_0.5Ta_0.5O₃ at 100 °C. The as-prepared SrFe_0.5Ta_0.5O₃ had the high crystallinity with particle size of ca. 31 nm. Under visible light irradiation, the pure SrFe_0.5Ta_0.5O₃ photocatalyst exhibited the photocatalytic NO removal efficiency of ca. 35.1% which is much higher than that of Sr₂FeTaO₆. We found that the nanoscale size, high crystallinity, and purity of as-prepared SrFe_0.5Ta_0.5O₃ particle can significantly increase the transfer and separation efficiency of the photogenerated charges from the bulk phase to surface. We also confirmed that the interaction between precursor cations and oleate ion was the key factor to decrease the solvothermal temperature and control the particle size. This work provided a novel strategy for preparing the Ta-containing perovskite photocatalyst under mild conditions.