Zirconium alloys are extensively utilized as fuel element materials in water-cooled nuclear reactors owing to their small thermal neutron absorption cross-section, exceptional resistance to high-temperature and high-pressure water corrosion, favorable compatibility with UO₂, and moderate mechanical properties. Loss of coolant accidents (LOCAs) pose a critical risk during the operation of nuclear reactors. During such accidents, the zirconium alloy cladding may be exposed to a mixed atmosphere of air and steam, undergoing high-temperature oxidation that could compromise its structural integrity and threaten nuclear reactor safety. Therefore, understanding the oxidation behaviors of zirconium alloys in high-temperature air-steam environments is essential. This study focused on Zr-0.75Sn-0.35Fe-0.15Cr-xNb alloys (x = 0, 0.15, 0.30, 0.50, and 1.0; mass fraction, %), which were smelted and formed into plate samples. The oxidation behaviors of these alloys in a mixed atmosphere comprising 20% air and 80% steam at temperatures ranging from 800 ^oC to 1200 ^oC were investigated using a synchronous thermal analyzer under simulated LOCA conditions. The microstructure and distribution of N and O in the cross-section of the oxidized samples were examined via OM, SEM, and electron probe microanalysis coupled with wave-dispersive spectroscopy. Results indicate that the effect of Nb content on the high-temperature oxidation behavior of the zirconium alloys is complex and does not directly correlate with changes in Nb content. In general, adding Nb may reduce the high-temperature oxidation resistance of Zr-0.75Sn-0.35Fe-0.15Cr-xNb alloys. The oxidation kinetics curves of the five alloys predominantly follow parabolic-linear or linear laws and display variations with changes in oxidation temperature and Nb content. In particular, oxidation transitions occur at 1000 and 1200 ^oC. In high-temperature steam containing air, the oxidation of zirconium alloys is considerably accelerated by the presence of N₂ and O₂ in air, with N serving as a “catalytic-like” agent, providing new oxidation pathways. The formation and subsequent reoxidation of ZrN contribute to the creation of porous oxide layers, undermining the protective capability of the oxide film. The changing content of Nb during the oxidation process influences the α↔β phase transformation in the zirconium alloy matrix and the monoclinic (m)↔tetragonal (t) phase transformation in the oxide film. Furthermore, Nb tends to increase the O solid solubility in α-Zr, and Nb oxidation promotes cracking of the oxide film, which detrimentally affects the oxidation resistance of the zirconium alloys. Conversely, an increase in Nb content reduces anion vacancy concentration and impedes O diffusion when Nb combines with O, thereby enhancing the oxidation resistance of the zirconium alloys.