Photoreforming of lignin has been explored as a fascinating technology to generate clean hydrogen energy and value-added aromatic monomers from biomass. However, its upscaling is impeded by unsatisfactory selectivity due to the lack of mechanistic investigations in the uncontrollable reaction pathways. Herein, we successfully controlled the concentration and position of sulfur vacancies within the ultrathin ZnIn2S4 nanosheets to optimize the photo-driven lignin model reforming process. The competition of proton transfer between the hydrogen evolution and dissociation of the β-O-4 linkage in the model compound of lignin was identified, and the modulation of the proton migration pathway was realized through S vacancy engineering in ZnIn2S4 nanosheets toward target products. As such, excellent selectivity for hydrogen and chemical monomers was achieved with a high concentration of S vacancies in the bulk and on the surface of ZnIn2S4, respectively. This study endows new mechanistic insights into the biomass photoreforming process and elucidates the structure/chemistry-catalysis correlation of ZnIn2S4 photocatalysts, which are beneficial for photocatalyst design and rational solar fuel production.