Abstract: Zirconium alloys are widely used as nuclear fuel cladding materials owing to their favorable neutron economy and excellent corrosion resistance. However, under high-temperature mixed oxygen-nitrogen atmosphere oxidation conditions, such as those encountered during accident scenarios or spent fuel management, the rapid degradation of the oxide layer can severely compromise cladding integrity and complicate subsequent recycling and reprocessing operations. Although extensive efforts have been devoted to oxidation kinetics and macroscopic degradation behavior, a comprehensive understanding of the multiscale microstructural evolution of the oxide layer and its correlation with oxidation kinetics remains limited. In this study, the oxidation behavior of zirconium alloy was systematically investigated over a temperature range of 850-1 100 ℃ under controlled oxygen-nitrogen atmospheres. Thermogravimetric analysis was combined with multiscale characterization techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron backscatter diffraction (EBSD), to establish a correlation between oxidation kinetics and oxide-scale microstructure. The results show that oxidation at 850-1 000 ℃ exhibits a characteristic four-stage behavior, including rapid formation of a thin non-protective oxide layer during heating, diffusion-controlled parabolic growth, stress-induced acceleration associated with crack formation, and a final saturation stage. In this temperature range, the oxide scale becomes progressively unstable due to stress accumulation and crack propagation, leading to deviations from parabolic kinetics. In contrast, at 1 100 ℃, the oxidation behavior differs significantly, with a dense and continuous oxide layer forming rapidly, exhibiting a bilayer morphology with an outer fine-grained region and an inner columnar-grained region. EBSD analysis reveals that the columnar ZrO₂ grains in the inner layer display a pronounced 〈001〉 preferred orientation along the growth direction, reflecting a strong crystallographic anisotropy during oxide growth. This oriented growth behavior suggests that competitive grain growth under diffusion control governs the development of the columnar structure. With increasing oxidation time, longitudinal microcracks gradually develop due to growth-induced stresses. Although these cracks locally modify oxygen transport paths, the overall oxidation kinetics remain parabolic, confirming that bulk diffusion through the dense oxide layer continues to dominate. Nanoscale TEM observations reveal severe intragranular lattice distortion and orientation fluctuations, indicating a highly strained oxide microstructure. Nb-rich second-phase particles (SPPs) at grain boundaries exhibit delayed oxidation behavior, leading to localized chemical and structural heterogeneities. These heterogeneities promote stress concentration and facilitate the nucleation of microvoids and microcracks, thereby progressively weakening the protective capability of the oxide layer. The combined effects of lattice distortion, second-phase particle behavior, and microcrack formation highlight the intrinsic microstructural complexity of the oxide scale. By integrating oxidation kinetics with multiscale microstructural analysis, this study establishes a mechanistic correlation between oxidation behavior and microstructural evolution. The results provide critical insights into the microstructural origins of oxide layer instability and high-temperature oxidation failure of zirconium alloy cladding. These findings offer valuable guidance for predicting cladding degradation behavior and for optimizing recycling and reprocessing strategies of zirconium alloy materials in nuclear fuel cycle applications.