Interface engineering is one of the key strategies to modify the intrinsic electronic structures of catalysts and consequently improve the electrochemical activity of the oxygen reduction reaction (ORR) in the cathodes of the energy conversion devices such as fuel cells and metal-air batteries. Herein, we proposed the mixed-phase mullite (AxSm1−xMn2O5-δ, x = 0–0.5, A=Ca, Sr, Ba), prepared by facile one-step co-precipitation method, to catalyze the oxygen reduction reaction (ORR). The X-ray diffraction (XRD) spectra show that each mixture includes three phases, i.e., mullite SmMn2O5, O-deficient perovskite AMnO3-δ and MnOx. Atomic bonding interfaces are formed between SmMn2O5 and AMnO3-δ, based on the observations of the high resolution transmission electron microscopy (HRTEM). Among these different mixed-phase samples, we find that BaxSm1−xMn2O5-δ/C exhibits the best ORR catalytic activity with the half-wave potential ~ 0.79 V (vs. RHE) and the highest stability over 20,000 s. This performance can be ascribed to the largest charge transfer from BaMnO2.83 to SmMn2O5. Subsequently, partial Mn4+ in mullite SmMn2O5 phase are reduced to active sites Mn3+ to achieve the e g unit occupancy in the interfacial depletion region. In comparison with the reactions over pure-phase mullite SmMn2O5, these transferred electrons are involved into ORR and thus accelerate the proceeding. Our work thus provides insights into designing heterogeneous compound catalysts via interfacing engineering in the applications of the electrochemical oxygen reactions.