Challenges like hydrogen evolution reaction (HER) and dendrite formation on zinc anodes hinder the practical application of aqueous zinc batteries (AZBs), primarily due to the lack of an exceptional passivating solid-electrolyte interphase (SEI). To effectively prevent water decomposition, it is essential to in-situ construct SEI prior to water reduction. This study shifts the focus from aprotic to protic SEI chemistry and predicts reduction potentials of mild protic compounds using thermodynamics. Based on this insight, mild protic 2-mercapto-1-methylimidazole (MMI) is introduced and found to be reduced prior to water at higher electrode potentials, as MMI exhibits slightly higher proton activity than water due to its thiol group. The reduction of MMI produces isothiocyanate species via a proposed electrocatalytic ring cleavage mechanism, which deposit on zinc anodes to form an SEI predominantly composed of organic compounds supplemented with Zn(OH)2. Benefiting from the unique advantage of preferential reduction before water, the MMI-derived SEI exhibits a superior passivating effect, as evidenced by a two-order-of-magnitude reduction in corrosion current of zinc anodes from 1170 μA cm-2 to 13.6 μA cm-2. The SEI effectively suppresses HER, and ensures dendrite-free zinc deposition, leading to high reversible zinc anodes with exceptional electrochemical performances.