Abstract: High-quality medium and high carbon steels, such as spring steel, impose stringent control requirements on the type and thickness of decarburized layers. In this work, isothermal oxidation and decarburization experiments were conducted on 28SiMnB spring flat steel within the temperature range of 700-1 000 ℃. The morphologies of oxide and decarburized layers were analyzed, and the coupling mechanism of oxidation and decarburization was investigated. The interface between the inner and outer scale layers was identified via Pt calibration, and EDS analysis confirmed that the inner scale consists of a multi-component oxide of Fe, Si and Cr, while the outer scale presents a three-layer structure composed of Fe₃O₄, FeO and a Fe₃O₄+FeO mixed phase. No decarburized layer was observed at the heating temperature of 700 ℃, whereas partial decarburized layers were detected in the specimens at 800 ℃, 900 ℃ and 1 000 ℃, with the thicknesses of 290 μm, 360 μm and 470 μm, respectively. However, a complete decarburized layer was not found, which may be attributed to the consumption of the decarburized layer by internal oxidation as well as the coupling effect of oxidation and decarburization.