Abstract: As well known in the petroleum industry and academia, Ni/ZnO catalysts have excellent desulfurization performance. However, the sulfur transfer mechanism of reactive adsorption desulfurization (RADS) that occurs on Ni/ZnO catalysts remains controversial. Herein, a periodic Ni nanorod supported on ZnO slab was built to represent the Ni/ZnO system, and density functional theory calculations were performed to study the sulfur transfer process and the role of H2 within the process. The results elucidate that the direct solid-state diffusion of S from Ni to interfacial oxygen vacancies (Ov) is more favorable than the hydrogenation of S to SH/H2S on Ni and the subsequent H2S desorption, and accordingly, H2O is produced on Ni rather than on ZnO. Ab initio thermodynamics analysis shows that the hydrogen atmosphere applied in preparing Ni/ZnO catalysts greatly promotes the Ov formation on ZnO surface, which accounts for the presence of interfacial Ov in freshly prepared catalysts. Under RADS condition, hydrogenation of interfacial O atoms to form O−H groups facilitates the reverse spillover of these lattice O atoms from ZnO to Ni, accompanied with the interfacial Ov generation. In contrast to the classic S transfer mechanism via H2S, the present work clearly demonstrates that the interfacial S transfer is a feasible reaction pathway in the RADS mechanism. More importantly, the existence of interfacial Ov is an essential prerequisite for this interfacial S diffusion, and H2 plays a key role in facilitating the Ov formation.