Zirconium alloy cladding will undergo high-temperature steam oxidation in a loss of coolant accident to make it be brittle and rupture due to absorbing oxygen, which will affect the safe operation of nuclear reactors. The high-temperature steam oxidation behavior of Zr-xSn-0.35Fe-0.15Cr (x=0.5, 0.75, 1.0, 1.2 and 1.5, wt.%) alloys at 800~1200 ℃ was studied by a synchronous thermal analyzer equipped with a steam generator. The cross-sectional microstructures of the samples after high-temperature steam oxidation were observed by metallographic microscope, and the O content was tested by electron probe microanalyzer. Results show that the high-temperature steam oxidation resistance and oxidation kinetics of zirconium alloys show a certain regularity with Sn content at different temperatures, which is mainly related to the action mechanism of α-Zr?β-Zr and m-ZrO2?t-ZrO2 phase transformation behaviors of zirconium alloys. As the oxidation temperature increases, the oxidized alloy samples present a double-layer structure of ZrO2 and α-Zr(O), accompanied by the appearance and disappearance of the mixed layer structure of β-Zr+α-Zr(O), which is caused by the effect of O on the α?β phase transformation. The increase in Sn content inhibits the diffusion of O from α-Zr→β-Zr. From the perspective that the increase in Sn content affects the α-Zr?β-Zr phase transformation and inhibits the diffusion of O from α-Zr→β-Zr, the mechanism of the effect of Sn content on the high-temperature steam oxidation behavior of zirconium alloys at different temperatures is discussed.