A novel and highly efficient ternary H₂SrTa₂O₇/g-C₃N₄/Ag₃PO₄ heterojunction was fabricated via simple impregnation and ion exchange methods with active performance for the degradation of methyl orange (MO) under solar-like irradiation. The optimal result for the binary composite was obtained for a H₂SrTa₂O₇/g-C₃N₄-60 wt% system. The incorporation of Ag₃PO₄ led to a further improvement in the photocatalytic performance, as compared to the binary composite and their single components, with values of around 45.18, 44.17 and 38.23 times higher than those of pristine H₂SrTa₂O₇, g-C₃N₄ and H₂SrTa₂O₇/g-C₃N₄, respectively. It was found that MO solution (20 mg L⁻¹) could be photo-degraded within 8 min with 50 mg of the H₂SrTa₂O₇/g-C₃N₄/Ag₃PO₄ composite. This enhanced photocatalytic behavior can be mainly ascribed to the prominently promoted charge separation efficiency, the matched band-edge positions, the extension of the absorption range and the synergistic effect among H₂SrTa₂O₇, g-C₃N₄ and Ag₃PO₄. In addition, X-ray photoelectron spectroscopy and Raman spectroscopy results confirmed a strong interface interaction between H₂SrTa₂O₇, g-C₃N₄ and Ag₃PO₄, which greatly promoted the photocatalytic activity due to the improved separation of photoinduced charge carriers. This work presents a plausible mechanism via active species trapping experiments and electron spin resonance measurements demonstrated that photogenerated holes (h⁺) and superoxide radicals (˙O₂⁻) play a crucial role in the photodegradation reaction under visible light irradiation. Thus, the proposed synthetic method for the fabrication of H₂SrTa₂O₇/g-C₃N₄/Ag₃PO₄ with superior photocatalytic activity provides new insights for the design of ternary photocatalysts with an ideal heterojunction and wide spectral response.