Graphitic carbon nitride (g-C3N4), a new two-dimensional (2D) polymeric semiconductor, is considered as one of the most promising candidates for visible-light photocatalytic H-2 evolution. The synthesis of thin-layered g-C3N4 is a facile approach to enhancing its photocatalytic properties. Here, a novel, cost-efficient, and time-saving strategy is reported to obtain 2D g-C3N4. In this equipment, bulk g-C3N4 and thin-layered g-C3N4 (denoted as G-CN) assembled from gaseous molecules were obtained simultaneously. The thin-layered g-C3N4 possesses more negative conduction-band minimum (similar to 0.18eV) relative to the bulk counterparty, leading to stronger redox ability. What is more, the carrier mobility and separation efficiency are both improved. As a result, the water splitting performance and photodegradation efficiency for methylene blue on thin-layered g-C3N4 are dramatically improved. The H-2 evolution rate and half-time of photodegradation obtained by kinetic fitting reached 48.83molh(-1) and about 1.5h, which is much more superior to that of bulk g-C3N4. Generally, the present work may bring out new thinking to synthesize thin-layered 2D materials.