This work developed one effective approach to introduce both the N defect and borate group on polymer carbon nitride (PCN) by facilely immersing it in a NaBH4 aqueous solution, and simultaneously control their number by varying the NaBH4 concentration. The resultant defective borate-decorated PCN (denoted as X-B-PCN, X refers to the used NaBH4 concentration) shows highly efficient in removing the ppb-level NO from an air flow under visible light irradiation. The removal efficiency displays a volcano-like variation with X, and the peak one reaches 44.1% by 1.32-B-PCN. The comparative studies on the optical property, the yield of active radicals, and the oxygen adsorption behaviors of PCN and X-B-PCN reveal that the synergy of N defect and borate decoration promotes the light absorbance, charge carrier separation and molecular oxygen adsorption, all of which contribute to an enhanced generation of the O-center dot(2)- and (OH)-O-center dot radicals for a better photocatalytic performance. The relative contribution of N defect to borate group in this synergy also depends on the X value, but the borate group always takes the major role. The in-situ DRIFTS investigation reveals that the NO is converted to nitrate. Overall, our work offered one viable strategy to make full use of the defect and group decoration for photocatalytic performance optimization of the PCN-based materials.