Oxygen reduction reaction (ORR) is the core reaction of proton exchange membrane fuel cell(PEMFC), but its slow oxygen reduction kinetics constrains the energy conversion efficiency of the fuel cell. Single-atom catalysts (SACs) are considered to be effective catalysts for catalyzing the oxygen reduction reaction due to their high atom utilization, but their insufficient active sites and low 4e^- selectivity lead to low catalytic efficiency. Cu clusters with different sizes were stretcher-loaded onto the singleatom catalyst Cu@BN using density-functional theory (DFT) calculations to construct Cu_x/Cu@BN (x=4, 13) composite catalysts in order to investigate the effect of the loading of the clusters on the catalytic oxygen reduction efficiency of Cu@BN single-atom catalysts. The results show that there is a more pro-nounced orbital hybridization between the h-BN carrier and Cu, suggesting that the strong interaction be-tween them makes the catalyst more stable; the cluster-anchored Cu@BN enhances the adsorption and activation of O₂ from the active site Cu, with the largest O -O bond length elongation and the largest charge transfer for the adsorbed Cu₄/Cu@BN. It is concluded that the loading of Cu clusters can significantly improve the catalytic efficiency of Cu - based monoatomic catalysts，especially Cu₄/Cu@BN exhibits the best catalytic performance for the oxygen reduction reaction.