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[Display omitted] • Multifunctional Co atoms are doped on the twin Mn 0.3 Cd 0.7 S homojunction surface. • Co atoms regulate S electron property and build a fast H-migration channel at S sites. • H migration endows sufficient H to sustain HER and reduces the H 2 evolution barrier. • Co atoms on the cubic phase act as OER active sites to enhance oxidation activity. • The Z-scheme interfacial charge-transfer mechanism is achieved. Low kinetics of water dissociation, unfavorable hydrogen adsorption, and high water-oxidation barriers restrict the photocatalytic overall water splitting (OWS) efficiency over sulfides. Herein, multifunctional Co atoms are doped on the twin Mn 0.3 Cd 0.7 S homojunction surface for efficient photocatalytic hydrogen (H 2) and oxygen (O 2) evolution. An ultrafast hydrogen atom (H*) migration channel bridging the water dissociation/H 2 evolution step is established between cubic-S and hexagonal-S sites by introducing Co atoms, facilitating the HER kinetics. Specifically, Co atoms regulate the electron properties of neighboring S atoms and weaken their charge accumulation, which promotes cubic-S sites to capture sufficient H* from the water dissociation zone and accelerates hexagonal-S sites to release H 2 through the H* migration channel. Meanwhile, unsaturated Co atoms on the cubic phase as oxygen evolution reaction (OER) sites promote water deprotonation and optimize the kinetics of the rate-determining step to form OOH* intermediate, enhancing water oxidation activity. Co atoms strengthen the internal electric field of the homojunction and local polarized electric field by increasing the work function difference between the two phases and introducing the high spin polarization, inhibiting the bulk and surface recombination of photogenerated carriers. The Co 0.01 Mn 0.29 Cd 0.7 S exhibits visible-light-driven H 2 and O 2 evolution rates of 443.12 and 219.67 μmol g−1 h−1, about 50 times that of the Mn 0.3 Cd 0.7 S. This work provides some guidance for the design of homojunction photocatalysts with efficient OWS performance. [ABSTRACT FROM AUTHOR]