Peracetic acid is widely used oxidizing agent for disinfection, water treatment, and chemical synthesis, but their conventional production relies on concentrated hydrogen peroxide and strong mineral acids, creating safety risks associated with storage and transport. Here, we report an electrocatalytic flow-cell strategy for the on-demand synthesis of peracetic acid (PAA) using a solid electrolyte reactor that couples in situ hydrogen peroxide generation with acid-catalyzed peracid formation. The system integrates a gas diffusion carbon cathode with a sulfonated solid electrolyte, enabling localized oxidant generation and rapid interfacial conversion. Under optimized conditions (320 mA applied current and 0.352 mL min⁻¹ electrolyte flow rate), the reactor produces approximately 300 ppm PAA, significantly exceeding the performance of conventional H-cell systems. Mechanistic experiments show that PAA formation is governed by the coupling between two-electron oxygen reduction and localized acidic sites within the solid electrolyte. Fluorescence microscopy reveals the spatial distribution of hydrogen peroxide across the electrolyte layer, while molecular dynamics simulations indicate that the protonation state of sulfonic groups regulates local hydration structures and reactant accessibility. These results highlight the importance of interfacial reaction environments in electrochemical peracid synthesis and demonstrate a promising route toward safe and decentralized oxidant production.