Single-atom electrocatalysts (SACs) have demonstrated fascinated electrocatalytic performance towards oxygen reduction reaction (ORR), while designing SACs with abundant accessible active sites is still challenging. Here, we report a facile sol-gel pore-sealing strategy for anchoring N coordinated Ru single atoms (SAs) onto the pore edges of the activated carbon spheres (Ru-SAs@N-ACSs) to enhance their accessibility. The fabricated electrocatalyst possesses outstanding ORR performance under alkaline conditions, outperforming most of recently reported SACs. Theoretical calculations screen that carbon edge could significantly modify the second coordination spheres of the central Ru atoms and the electronic structures of Ru-N4 sites, contributing to preferable ORR intermediate adsorption energy and high intrinsic catalytic activity. The Zn–air battery assembled using Ru-SAs@N-ACSs exhibits high power density and remarkable charge–discharge stability. The projected strategy provides valuable guidance on fabricating highly accessible SAs and engineering the second coordination spheres of metal atoms to achieve efficient SACs.