Graphitic carbon nitride (g-C3N4) with nitrogen vacancies (CN-x) was fabricated by a straightforward one-step N2H4 center dot H2O-assisted thermal polymerization method, avoiding high-energy consumption and complex posttreatment. 100-fold precursor amplification experiment proved that such a simple one-step method could achieve scalable synthesis. Experiment results show that nitrogen vacancies, introduced into CN-x architecture by removing N atoms in the C-containing triazine rings (C=N-C) (N-2C ) sites, cause bandgap narrowing and generate downward-shifted midgap states under the conduction band (CB) edge, which consequently extend the visible-light absorption and inhibit recombination of electrons and holes. As a result, under optimal amount of N2H4 center dot H2O added, CN-75 shows a highest H-2 evolution rate of 8171.4 mu mol.h(-1).g(-1) and 3895.1 mu mol.h(-1).g(-1) under lambda > 420 nm and lambda > 470 nm visible-light irradiation, respectively, far higher than that of pristine g-C3N4 (481.6 mu mol.h(-1).g(-1), lambda > 420 nm and 148.6 mu mol.h(-1).g(-1), lambda > 470 nm). Moreover, stability assays show that the as-prepared CN-75 possesses satisfactory light and structure stability after ten cycling runs (4 h per cycling). This work offers a simple and effective route for fabricating high-performance nitrogen vacancies modified g-C3N4 photocatalyst on a large scale.