Atomically dispersed Fe‒N4 catalysts are proved as promising alternatives to commercial Pt/C for the oxygen reduction reaction. Most reported Fe‒N4 catalysts suffer from inferior O‒O bond-breaking capability due to superoxo-like O2 adsorption, though the isolated dual-atomic metal sites strategy is extensively adopted. Atomic Fe clusters hold greater promise for promoting O‒O bond cleavage by forming peroxo-like O2 adsorption. However, the excessively strong binding strength between Fe clusters and oxygenated intermediates sacrifices the activity. Here, we first report a Fex/Cu‒N@CF catalyst with atomic Fe clusters functionalized by adjacent single Cu‒N4 sites anchoring on porous carbon nanofiber membrane. The theoretical calculation indicates that the single Cu‒N4 sites can modulate the electronic configuration of Fe clusters to reduce O2* protonation reaction free energy which ultimately enhances the electrocatalytic performance. Particularly, the Cu‒N4 sites can increase the overlaps between d orbitals of Fe and p orbitals of O to accelerate O‒O cleavage in OOH*. As a result, this unique atomic catalyst exhibits a half potential (E1/2) of 0.944 V in an alkaline medium exceeding that of commercial Pt/C, whereas acidic performance E1/2 = 0.815 V is comparable to Pt/C. This work shows the great potential of single atoms for improvements of atomic cluster catalysts.