Efficient separation of photoexcited charge is the bottleneck of photocatalysis in the practical application. Design and construction of the heterojunction with proper interface and rational electronic structure is urgently needed to overcome this limitation. Here, we propose a conceptual design of in-plane heterostructure with continuous pi-conjugated bond through the incorporation of reduced graphene (rGO) into graphitic carbon nitride (g-C3N4). The unique in-plane heterostructural rGO/g-C3N4 nanosheet realizes high photogenerated electron-hole pair separation efficiency through the pi-conjugated build-in electric field at the interface. Incorporation of rGO endows the photocatalyst a wider photoresponse window and better oxidizability, beneficial for light absorption and redox reaction. Consequently, the in-plane rGO/g-C3N4 heterostructure can enhance photocatalytic water purification performance towards different kinds of pollutants (such tetracycline, beta naphthol, rhodamine B, etc.). Notably, the photocatalytic rate of 5 wt% rGO/g-C3N4 are estimated to be 3.89 (tetracycline), 5.17 (beta naphthol), and 6.00 times (rhodamine B) higher than g-C3N4, respectively. This new insight of interface incorporation engineering helps to inspire innovative semiconductor structural designs with fast charge transfer and satisfied photocatalytic activity.