Mn-based oxygen-deficient perovskite catalysts A2Mn2O5 (A = Ca, Sr) have been experimentally proved high oxygen evolution reaction (OER) activities for replacing Pt in oxygen electrocatalysis. Nevertheless, the correlation between the fundamental electronic structure at room temperature and the corresponding electrocatalysis is not fully accessible. In this paper, we combine the ground state density functional theory (DFT) method and dynamic mean-field theory (DFT+DMFT) at room temperature to investigate the origin of the OER difference for electrocatalysts A2Mn2O5 (A = Ca, Sr). We find that at room temperature the highest occupied Mn dz2 orbital in the square pyramidal crystal field of oxygen-deficient perovskites A2Mn2O5 with insulating properties can provide a moderate bonding strength with intermediate hydroxyl OH*, leading to a high OER catalytic activity. According to the electronic structure analysis, we observe that replacing the A-site element Ca by Sr with the larger ionic radii would result in a higher OER activity due to the weakened hybridization between the Mn dz2 orbital and the O pσ orbital of OH*. This insight could provide hints for the screening metal oxide electrocatalysts in the applications of the energy storage and conversion.