During loss of coolant accident, the oxidation can cause burst of the cladding tube in high temperature steam, which lead to the leakage of the nuclear fuel. At present work, the oxidation behaviors of a Zr-Sn-Nb alloy were investigated in 1000~1250℃ steam. The weight gains per area were calculated by the weight gain method. The microscopic morphology, thickness of the alloy were measured by a scanning electron microscope. The oxidation kinetics of the alloy was characterized by weight gain and the thickness increase of α-Zr (O) and ZrO₂ layers. The results show that the parabolic weight gain curves and ZrO₂ layer growth kinetics curves become to be linear in 1000℃ steam for about 1500s, while α-Zr (O) growth kinetics always follow the parabolic law. Meanwhile, a large number of cracks are formed in the ZrO₂ layers. The weight gain kinetics curves and α-Zr (O) and ZrO₂ layers growth kinetics curves all follow the parabolic law in 1100~1250℃ steam,and the ZrO₂ layer is almost intact. The oxidation resistance of Zr-Sn-Nb alloy is higher than that of Zr-4 alloy. The growth rate of ZrO₂ layer and α-Zr (O) layer is slower than that of Zr-4 alloy.