The degradation of aquatic pollutants using eco-friendly and non-toxic photocatalytic materials is a pivotal strategy for water pollution remediation. However, single-component photocatalysts typically suffer from low photocatalytic efficiency due to limited light absorption spectra and rapid recombination of photogenerated charge carriers. This study reports a novel and facile one-step mixing strategy for realizing triple synergistic modifications: heterostructured composite construction, specific surface area regulation, and efficient photogenerated electron–hole pair separation of Bi₂MoO₆ (BMO) via composite enhancement with low-cost and intrinsically green g-C₃N₄ (CN), which avoids the high cost, complex processes, and potential pollution risks of precious metal/heavy metal modification for BMO. Under visible-light irradiation, the BMO composite modified with 15 wt% CN achieved a dye removal rate of 85.1% within 60 min, representing a 1.6-fold enhancement in photocatalytic performance compared with that achieved using pristine BMO. We further clarify the unique photocatalytic mechanism of the CN/BMO heterojunction via radical quenching experiments, identifying photogenerated holes (h⁺) and superoxide radicals (·O₂⁻) as the dominant active species for Rhodamine B (RhB) degradation. This study systematically demonstrates a scalable photocatalyst preparation method that integrates controllable specific surface area, rational heterostructure construction, and simple operation, and we provide an in-depth investigation into the photocatalytic reaction process and underlying synergistic enhancement mechanism. The proposed non-metallic modification route provides a new theoretical and experimental basis for the design of high-efficiency BMO-based photocatalysts, and the as-prepared CN/BMO composite holds great potential for practical application in sustainable solar-driven water purification.