Zr-0.75Sn-0.35Fe-0.15Cr-xNb (x=0, 0.15, 0.50, 1.00, wt.%) alloys were smelt and parpared to plate specimens a in this paper. The high temperature steam oxidation behavior of four zirconium alloys at 800~1200 ℃ under simulated LOCA conditions was studied, and the cross-sectional microstructure was analyzed by metallographic microscope. The results show that the gain weight after oxidation of the four zirconium alloys does not have consisitent trend with the change of Nb content, and the oxidation kinetics is mostly linear. Only the oxidation kinetics of the alloy with 0.5% Nb at 1000 ℃ has two transitions, that is, from linear law to power exponential law and then to linear law. The transformation temperature of the matrix in zirconium alloy decreases with the increase of Nb content, while the phase transition behavior of its oxides does not show consisitent trend with the change of Nb content, it shows that the influence of Nb content on the phase transformation behavior of zirconium alloy and oxide is more complicated than that of the alloy. When the oxidation temperature is 800 ℃, 1000 ℃ and 1200 ℃, the cross-sectional structure of the oxidized sample is: ZrO2 and α-Zr(O), ZrO2, α-Zr(O) and prior β-Zr layer, ZrO2 and α-Zr(O). The thickness ratio of each layer of the cross section of the 800 ℃ oxidized sample does not change with the Nb content. The proportion of the thickness of the α-Zr(O) layer in the cross-section of the oxidized sample at 1000 ℃ increases with the increase of the Nb content. The proportion of the thickness of the prior β-Zr layer is just the opposite, and there is a finger-like intrusion of α-Zr(O). The ratio of the thickness of the microstructure of the cross-section of the oxidized sample at 1200 ℃ varies with the Nb content more complicated. It shows that Nb can promote the transformation of β→α-Zr(O).