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A diesel oxidation catalyst (DOC) is installed upstream of an exhaust after-treatment line to remove CO and hydrocarbons and generate NO 2 . The catalyst should possess both good oxidation ability and thermal stability because it sits after the engine. We present a novel high-performance DOC with high steam resistance and thermal stability. A selective dissolution method is adopted to modify the surface physicochemical environment of CeO 2 -SmMn 2 O 5 . The X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, Raman, electron paramagnetic resonance, hydrogen temperature-programmed reduction, and temperature-programmed desorption results reveal that surface Sm cations are partially removed with the exposure of more Mn 4+ and Ce 3+ cations and the presence of active surface oxygen species. This mechanism benefits the oxygen transformation from Ce to Mn and promotes the Ce 3+ + Mn 4+ ↔ Ce 4+ + Mn 3+ redox cycle according to the in situ near-ambient pressure X-ray photoelectron spectroscopy and in situ diffuse reflectance infrared Fourier transformation spectroscopy results. Under laboratory-simulated diesel combustion conditions, the catalyst demonstrates excellent low-temperature oxidation catalytic activity (CO and C 3 H 6 conversion: T 100 = 250 °C) compared to a Pt-based catalyst (CO and C 3 H 6 conversion: T 100 = 310 °C) with a WHSV of 120,000 mL g -1 h -1 . Specifically, NO conversion reaches 68% when the temperature is approximately 300 °C.