© 2020 The Royal Society of Chemistry. Black phosphorus has attracted ever-growing interests as an abundant and low-cost nonmetal photocatalyst for renewable energy generation. To achieve high efficiency for practical implementation, it is imperative to understand the regulation for tuning the chemical activity and meanwhile suppressing photocarrier recombination in black phosphorus. Here for the first time, we exploit the typical point defects in phosphorene to activate its basal plane and optimally modulate the photocarrier dynamics for solar-driven nitrogen reduction reaction (NRR). Combining first-principles calculations and time-dependent ab initio nonadiabatic molecular dynamics simulations, we elucidate the exact influence of each type of defect on the reduction capability and lifetime of photogenerated carriers as well as the NRR activity and selectivity of phosphorene. These results provide essential knowledge for precise design of high-efficiency nonmetal photocatalysts for hydrogen fuel storage at the atomic level.