Reaction atmosphere of preparing g-C3N4 can affect the photocatalytic activity in the electronic structure and the separation efficiency of photo-induced charge carriers. However, in the process of preparing graphitic carbon nitride, it is still a challenge to introduce an atmosphere by a simple and economic thermal-treatment method. Herein, a simple approach to preparing g-C3N4 nanosheets under generated gas molecules in a one-step calcination (CN-A) was reported. Such a one-step calcination method not only saved energy consumption, but also provided an economical way to combine a variety of favorable gases. The CN-A with more nitrogen vacancies significantly improved the separation efficiency of photo-induced electron-hole pairs. Furthermore, it formed small grain and high porosity with large surface areas. As expected, the CN-A exhibited a hydrogen evolution rate of 1198 mu mol g(-1) h(-1) under visible-light irradiation, which was 2.5 times higher activity than the g-C3N4 calcined under nitrogen atmosphere (CN-N), as well as higher than most of the reported bulk g-C3N4. Our research provides a new insight into the reaction atmosphere employed in g-C3N4 materials in modifying chemical structure, grain size, porosity and photophysical properties.