Graphitic carbon nitride (g-C3N4), liking a yolk-shell cake, was successfully prepared via a gas-bubble template method with the assistance of NH4Cl. In this special structure, the shell region consists of numerous thin sheets with rich porosity, while the yolk region consists of a lot of cross-linked large hollow particles. The formation mechanism of yolk-shell structure was investigated via adjusting the mass ratio of NH4Cl to melamine, and calcination temperature. The surface area and pore volume of the two types of porous g-C3N4 nanomaterials significantly depend on the mass ratio of NH4Cl to melamine in a certain extent. The as-prepared porous g-C3N4 nanomaterials exhibit improved photocatalytic activity compared to bulk g-C3N4. Moreover, the porous thin sheets in shell region possess larger specific surface area, higher light utilization, higher charge separation and transfer efficiency, than the hollow particles in yolk region, leading to the superior photocatalytic performance. We believe that this simple, economical, and nontoxic approach for the preparation of g-C3N4 nanomaterials opens new possibilities for scalable synthesis of g-C3N4-based materials with more controllable structures.