Direct nitrogen photofixation is a feasible solution toward sustainable production of ammonia under mild conditions. However, the generation of active sites for solar-dirven nitrogen fixation not only limits the fundamental understanding of the relationship among light absorption, charge transfer, and catalytic efficiency but also influences the photocatalytic activity. Herein, we report two-dimensional boron-doped niobic acid nanosheets with oxygen vacancies (B-V-o-HNbO3 NSs) for efficient N-2 photofixation in the absence of any scavengers and cocatalysts. Impressively, B-V-o-HNbO3 NS as a model catalyst achieves the enhanced ammonia evolution rate of 170 mu mol g(cat)(-1) h(-1) in pure water under visible-light irradiation. The doublet coupling representing (NH4+)-N-15 in an isotopic labeling experiment and in situ infrared spectra confirm the reliable ammonia generation. The experimental analysis and density functional theory (DFT) calculations indicate that the strong synergy of boron dopant and oxygen vacancy regulates band structure of niobic acid, facilitates photogenerated charge transfer, reduces free energy barriers, accelerates reaction kinetics, and promotes the high rates of ammonia evolution. This work provides a general strategy to design active photocatalysts toward solar N-2 conversion.