Perovskite oxides are of particular interest for the oxygen evolution reaction (OER) due to their high intrinsic activity. However, low surface area and nonpores in bulk phase generally limit the mass transport and thereby result in unsatisfactory mass activity. Herein, we propose a “molecular-level strategy” with the simultaneous modulation of the ordered pores on the oxygen-deficient sites along with sulfur (S) substitution on oxygen sites at the molecular level to boost the mass transport behavior of perovskite electrocatalyst for enhanced mass activity. As a proof of concept, the elaborately designed brownmillerite oxide Sr2Co1.6Fe0.4O4.8S0.2 (S-BM-SCF) shows approximately fourfold mass activity enhancement in 1 M KOH compared with the pristine SrCo0.8Fe0.2O3-δ (SCF) perovskite. Comprehensive experimental results, in combination with theoretical calculations, demonstrate that the intrinsic molecular-level pores in the brownmillerite structure can facilitate reactive hydroxyl ion (OH−) uptake into the oxygen-vacant sites and that S doping further promotes OH− adsorption by electronic structure modulation, thus accelerating mass transport rate. Meanwhile, the S-BM-SCF can significantly weaken the resistance of O2 desorption on the catalyst surface, facilitating the O2 evolution. This work deepens the understanding of how mass transport impacts the kinetics of the OER process and opens up a new avenue to design high-performance catalysts on the molecular level.