Porous g-C₃N₄ composed of nanosheets (p-gCN-NS) was synthesized through firstly grinding the hydrochloric acid-treated melamine with ammonium chloride (NH₄Cl) and then calcining the mixture at 550° C for 2 h. The photocatalytic properties were determined by degrading Rhodamine B (RhB) and tetracycline (TC) aqueous solution (10 mg/L, 100 mL). Results show that the crystallinity, size and thickness of g-C₃N₄ in p-gCN-NS reduced dramatically compared with those in g-C₃N₄ sample synthesized without the addition of NH₄Cl (gCN). Specific surface area of p-gCN-NS was 63.65 m²·g⁻¹, which was 6.7 times that of gCN. After photocatalysis for 50 min, the degradation rate of RhB achieved 99.32% by p-gCN-NS (100 mg), whose reaction rate constant (k = 0.09317 min⁻¹) was 10.6 times higher than that of gCN. p-gCN-NS also exhibited excellent photocatalytic activity for degrading colorless tetracycline aqueous solution. The significant improvement of photocatalytic performance can be credited to the reduced sized and thickness of g-C₃N₄, higher specific surface area and pore structure, as well as the weakened recombination rate of photogenerated electron–hole pairs in p-gCN-NS. This work provides a feasible and facile method for synthesis porous g-C₃N₄ composed of nanosheets with excellent photocatalytic performance.