Single-site metal atoms (SMAs) on supports are attracting extensive interest as new catalytic systems due to maximized atom utilization and superior performance. However, rational design of configuration-optimized SMAs with high activity from the perspectives of fundamental electron spin is highly challenging. Herein, N-coordinated Fe single-atoms are successfully distributed over axial carbon micropores to form dangling-FeN4 centers (d-FeN4). This unique d-FeN4 demonstrates much higher intrinsic activity towards oxygen reduction reaction (ORR) in HClO4 than FeN4 without micropore underneath and commercial Pt/C.