The high temperature steam oxidation behaviour of zirconium alloys under Loss of Coolant Accident (LOCA) is one of the issues that needs to be focused on. In this paper, we smelted Zr-xNb (x=0.5, 1.0, 1.5, wt.%) alloys and Zr-1Nb-yCr (y=0.05, 0.2) alloys and prepared them as plate samples. The oxidation behaviour of the five zirconium alloys in steam at 900~1200 °C under simulated LOCA conditions was investigated using a simultaneous thermal analyser. And the microstructure and microhardness of the samples before and after oxidation were studied using a metallographic microscope and a microhardness tester, respectively. The results show that the oxidation resistance to high-temperature steam of Zr-xNb alloys does not vary monotonically with the change of Nb content at 900~1100 °C, and changes with temperature; the addition of Cr makes the oxidation resistance to high-temperature steam of Zr-1Nb alloys worse, and the effect is complicated; overall, the Zr-1.5Nb alloy has the best performance at 900~1100 °C. When oxidized in steam at 1200 °C, the addition of Nb and Cr has little effect on the oxidation resistance to high temperature steam of the alloys.With increasing temperature, the oxidation kinetic of the five alloys undergoes a parabolic → linear transition with multiple transitions. Finally, the mechanism that Nb and Cr affect the high temperature steam oxidation behaviour of zirconium alloys has been investigated from the following perspectives: the solid solution content of O in the Zr matrix, the α?β phase transition of the Zr matrix and the monoclinic (m) ? tetragonal (t) phase transition of the oxide film.