The high-temperature steam oxidation behavior of zirconium alloys is one of the issues that need to be focused on under loss of coolant accident (LOCA). To study the effect of Sn on the high-temperature steam oxidation behavior of zirconium alloys, SH12(Zr-0.75Sn-0.2Fe-0.1Cr, wt.%) and Zr-4(Zr-1.5Sn-0.2Fe-0.1Cr) alloys with different Sn contents were used for high-temperature steam oxidation tests at 1000, 1050, 1100 and 1200 ℃, under simulated LOCA conditions. The microstructure of the oxidized samples and the microhardness of the samples before and after oxidation were analyzed by optical microscope, electron probe microanalyzer, and microhardness tester. The results show that SH12 with low Sn content has better oxidation resistance to high-temperature steam than Zr-4 alloy with high Sn. This indicates that reducing the Sn content can improve the oxidation resistance to high temperature steam of zirconium alloys. Before oxidation, the microhardness of the SH12 alloy is lower than that of the Zr-4 alloy, while after oxidation the microhardness of the SH12 alloy matrix is significantly higher than that of the Zr-4 alloy, which is consistent with the result that the oxygen content in the matrix of the SH12 alloy is higher than that of the Zr-4 alloy after oxidation. This indicates that Sn has the effect of inhibiting the diffusion of oxygen in the Zr matrix. Through first-principles calculations, it is found that Sn is easily combined with O, thereby inhibiting O diffusion in the zirconium matrix, which reasonably explains the higher oxygen content in the oxidized SH12 alloy compared to the Zr-4 alloy.