Direct and efficient photocatalytic water splitting is critical for sustainable conversion and storage of renewable solar energy. Here, we propose a conceptual design of two-dimensional C3N4-based in-plane heterostructure to achieve fast spatial transfer of photoexcited electrons for realizing highly efficient and spontaneous overall water splitting. This unique plane heterostructural carbon ring (Cring)-C3N4 nanosheet can synchronously expedite electron-hole pair separation and promote photoelectron transport through the local in-plane π-conjugated electric field, synergistically elongating the photocarrier diffusion length and lifetime by 10 times relative to those achieved with pristine g-C3N4. As a result, the in-plane (Cring)-C3N4 heterostructure could efficiently split pure water under light irradiation with prominent H2 production rate up to 371 μmol g-1 h-1 and a notable quantum yield of 5% at 420 nm.