<p>The electrochemical oxidation of propylene presents a promising strategy for propylene oxide (PO) synthesis, but is severely hindered by the complex reaction pathways and the low PO selectivity. In this work, a series of Ag-decorated FeOOH catalysts is designed to elucidate the reaction mechanism of the electrochemical propylene oxidation reaction for PO electrosynthesis. An optimal Faradaic efficiency of PO of 32.0% is achieved over the single-Ag-atom decorated FeOOH catalyst (Ag₁-FeOOH) at 2.4 V versus reversible hydrogen electrode. The in-situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy and in-situ ⁵⁷Fe Mössbauer spectroscopy measurements combined with density functional theory calculations reveal the dual-site synergistic catalytic mechanism for electrochemical propylene epoxidation over Ag₁-FeOOH, where single-Ag-atom sites catalyze water oxidation to generate reactive oxygen species, while the adjacent Fe sites serve as adsorption sites for propylene activation. This study provides clear insights into the dual-site synergistic electrocatalytic mechanism of propylene epoxidation and sheds light on the rational design of single-atom catalysts for electrochemical organic synthesis.</p>