A new Ag₃PO₄/nitridized Sr₂Nb₂O₇ (N: 0–6.18 wt%) heterojunction was designed to eliminate gaseous pollutants under visible light irradiation. The phase compositions, optical properties, and morphologies of the heterojunction photocatalysts were systematically investigated via powder X-ray diffraction, UV-visible absorption spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, and transmission electron microscopy. Calculations of the electronic structure indicated that the top of the valance band of Sr₂Nb₂O₇ could be raised by nitrogen doping. Therefore, the electronic structure of the Ag₃PO₄/nitridized Sr₂Nb₂O₇ composite photocatalysts could be continually changed by controlling the amount of nitrogen in nitridized Sr₂Nb₂O₇. Photocatalytic degradation of isopropyl alcohol (IPA) was carried out to test the photocatalytic activity of the heterojunction. The highest activity (CO₂ evolution rate, 10.32 ppm h⁻¹) was observed over the Ag₃PO₄/nitridized Sr₂Nb₂O₇ heterojunction prepared by nitridation of Sr₂Nb₂O₇ (SNO) at 1023 K. The CO₂ evolution rate over the heterojunction was about 40 times higher than that over pure Ag₃PO₄ (CO₂ evolution rate, 0.26 ppm h⁻¹) under visible light irradiation. An investigation of the energy-band structure via valence band X-ray photoelectron spectroscopy indicated that the conduction band (CB) and valence band (VB) of Ag₃PO₄ are both more positive than those of nitridized Sr₂Nb₂O₇, which facilitates the separation and transfer of photogenerated electrons and holes between the two photocatalysts. By continually adjusting the electronic structures, an optimal band gap for the nitridized Sr₂Nb₂O₇ of 2.15 eV was obtained, and the potential of the valance band was +1.88 eV.