The isothermal steam oxidation behavior of the Zr−Sn−Nb alloy at 1050 °C was studied. In this study, the oxidation weight gain of Zr−Sn−Nb samples with oxidation durations ranging from 100 s to 5000 s was calculated. The oxidation kinetic properties of the Zr−Sn−Nb alloy were obtained. The macroscopic morphology of the alloy was directly observed and compared. The microscopic surface morphology, cross-section morphology, and element content of the Zr−Sn−Nb alloy were analyzed using scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and energy disperse spectroscopy (EDS). According to the results, the cross-sectional structure of the Zr−Sn−Nb alloy consisted of ZrO2, α-Zr(O), and prior-β. During the oxidation process, its weight gain versus oxidation time curve followed a parabolic law. The thickness of the oxide layer increases. Micropores and cracks gradually appear on the oxide film. Similarly, the thicknesses of ZrO2and α-Zr versus oxidation time were in accordance with the parabolic law.