Vacancy engineering, that is, self-doping of vacancy in semiconductors, has become a commonly used strategy to tune the photocatalytic performances. However, there still lacks fundamental understanding of the role of the vacancies in semiconductor materials. Herein, the g-C$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$N$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$ nanosheets with tunable nitrogen vacancies are prepared as the photocatalysts for H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ evolution and CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ reduction to CO. On the basis of both experimental investigation and DFT calculations, nitrogen vacancies in g-C$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$N$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$ induce the formation of midgap states under the conduction band edge. The position of midgap states becomes deeper with the increasing of nitrogen vacancies. The g-C$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$N$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$ nanosheets with the optimized density of nitrogen vacancies display about 18 times and 4 times enhancement for H$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ evolution and of CO$\mathrm{<mrow\; data-mjx-texclass="ORD">2</mrow>}$ reduction to CO, respectively, as compared to the bulk g-C$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$N$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$. This is attributed to the synergistic effects of several factors including (1) nitrogen vacancies cause the excitation of electrons to midgap states below the conduction band edge, which results in extension of the visible light absorption to photons of longer wavelengths (up to 598 nm); (2) the suitable midgap states could trap photogenerated electrons to minimize the recombination loss of photogenerated electron–hole pairs; and (3) nitrogen vacancies lead to uniformly anchored small Pt nanoparticles (1–2 nm) on g-C$\mathrm{<mrow\; data-mjx-texclass="ORD">3</mrow>}$N$\mathrm{<mrow\; data-mjx-texclass="ORD">4</mrow>}$, and facilitate the electron transfer to Pt. However, the overintroduction of nitrogen vacancies generates deeper midgap states as the recombination centers, which results in deterioration of photocatalytic activities. Our work is expected to provide new insights for fabrication of nanomaterials with suitable vacancies for solar fuel generation.