The major limitation for the photodegradation of pollutants by g-C3N4-based and lead-free halide perovskite photocatalysts are the moderate adsorption-photocatalytic ability and interfacial charge transfer rates. Herein, we have synthesized boron doped and nitrogen deficient g-C3N4 (BNCNx/y) and fabricated the BNCNx/y/Cs3Bi2Br9 heterojunctions by anti-solvent method to regulate bandgap and improve photoabsorption. The optimal ratios BNCN0.6/450/Cs3Bi2Br9 (BNCN-CBB-30) exhibited the highest degradation efficiency of 98.62 % for chloroquine phosphate (CQ) within 60 min, which possessed not only high oxidation ability (>82.19 %) for a variety of antibiotics but excellent structural stability and reusability. DFT calculations revealed that BNCNx/y/Cs3Bi2Br9 had the shortest bond distance and greater adsorption energy with CQ. Under visible light irradiation, the rapid electron transfer from Cs3Bi2Br9 to BNCN450/0.6 driven by the built-in electric field enhanced generation of •O2–, 1O2, and •OH. This work provides new insights into the internal structure design of heterojunction photocatalysts.