The microstructure and isothermal oxidation kinetics of Zr-Nb-Cu alloy under water vapor environment in the temperature range 700°C to 1000°C were studied, and the evolution laws of oxide film macro-morphologies and microstructures were also analyzed. The results show that the weight gain curves of water vapor oxidation of the alloy between 700°C and 900°C are in accordance with parabolic law, and the curve deviates from the parabolic law at 1000°C. According to the oxidation weight gain curves, the oxidation activation energy Q, the expressions of the parabolic rate constant Kp and the weight gain W were calculated respectively. The oxide film structure analysis results indicate that the oxidation temperature is higher, the crack and bulge of oxide film is more obvious.