Abstract: To enhance the photocatalytic degradation performance of graphitic carbon nitride (g-C₃N₄) toward pollutants, ammonium carbonate was employed as a modifier to tune the microstructure of g-C₃N₄ via a one-step pyrolysis strategy. The structure, composition, and morphology of the modified g-C₃N₄ were characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), and scanning electron microscopy (SEM). Its photocatalytic degradation efficiency for aqueous pollutants was systematically evaluated. The results demonstrate that the sample modified with 0.2% ammonium carbonate exhibited optimal photocatalytic activity, showing significantly improved degradation performance for model pollutants such as Rhodamine B (RHB) and tetracycline. Specifically, the degradation efficiency for RHB increased by 2.36-fold, and the material retained approximately 90% of its initial degradation rate after four consecutive cycles. The incorporation of ammonium carbonate notably strengthened the π–π conjugation system of g-C₃N₄, enhanced its crystallinity and structural ordering, and optimized its electronic band structure. Furthermore, the modification increased the density of active sites on the material surface. This study provides a feasible approach for improving the photocatalytic degradation performance of g-C₃N₄-based materials toward water pollutants.