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Thermal stability of nanocrystalline grains is a crucial factor that determines the unique microstructure and properties of the gradient nanostructured (GNS) materials at elevated temperatures. Nevertheless, oxidation is unavoidable for GNS metal materials utilized at high temperatures, potentially impacting the microstructure stability. In this study, we reveal the correlation between the high-temperature selective oxidation and the thermal stability of GNS layer through experiments and phase-field simulations. The improved oxidation resistance of GNS samples was ascribed to the excellent thermal stability of (Cr, Mn)3O4 oxides and a large proportion of low-energy twin boundaries. After prolonged oxidation, the GNS layer exhibited a bimodal microstructure. To analyze the elemental diffusion mechanism and microstructure evolution in the GNS layer, the phase-field simulation technique was employed. Selective oxidation led to the concentration of chromium reduced in the grain-boundary region, thereby diminishing the thermal stability of the grains and causing abnormal grain growth in the surface layer. Particularly, grain growth had a cumulative effect, the topmost grains coarsening will cause grain growth in the underlying layers, and subsequently, the grains in the interior region will also be gradually affected.