In order to enhance the nitriding efficiency, 42CrMo steel was selected as the testing material. Plasma oxynitriding using different air flow in the same pulsed plasma nitriding system for 42CrMo steel was investigated innovatively, and compared with conventional plasma nitriding processing. The cross-sectional microstructures, phases, corrosion resistance were determined by optical microscopy, X-ray diffraction, microhardness tester and electrochemical workstations. The results show that plasma oxynitriding owns much higher efficiency compared with plasma nitriding, thus makes thicker compound layer under the same other conditions. Meanwhile, higher porosity of nitrided layer is obtained. Furthermore, corrosion resistance can be significantly improved due to chemically stable formation and compact Fe₃O₄ oxide in the compound layer during plasma oxynitriding process, and air flow of 0.3L/min offers the optimum performance for 42CrMo steel due to highest ratio of Fe₃O₄ to Fe₂O₃. This research can provide a reference for improving the surface modification process of 42CrMo.