The photocatalytic application of graphic carbon nitrate (g-C3N4) was limited due to the shortcomings of few exposed active sites and serious recombination rate of photo-generated carrier. In this paper, B-doped g-C3N4 nanosheets were synthesized by a two-step thermal polymerization method. As-prepared B-doped g-C3N4 nanosheets maintained the initial structure of g-C3N4. The composition analysis results indicated the successful introduction of B element into the g-C3N4 structure. Under visible light irradiation, as-prepared B-doped g-C3N4 nanosheets revealed excellent NO removal and hydrogen production performance, reaching 54% and 1639.29 μmol/g/h, respectively. The superior photocatalytic performance was attributed to the ultrathin nanosheets obtained by thermal exfoliation, which have higher specific surface area and more active sites. The introduction of B enhanced the charge migration and inhibits the recombination of photogenerated carriers. The cyclic experiment indicated that the sample exhibited high stability. Enhanced photocatalytic mechanism from B doping was discussed.