Abstract: The corrosion experiment of a new Zr-0.4Sn-0.7Nb-0.3Fe-0.1Cr-0.15Mo-0.12O low-tin medium-niobium zirconium alloy in the environment containing dissolved oxygen with mass concentration of 300, 1 000 μg · L⁻¹ for 300 d was carried out in an external autoclave. The microstructure change of oxide film on alloy surface during corrosion was studied. The electrochemical properties of the alloy after corrosion were characterized by off-site electrochemical detection. The effect of dissolved oxygen on the electrochemical properties was analyzed. The results show that during corrosion, in the environment containing dissolved oxygen, the thickness of oxide film on alloy surface increased linearly with time. The number of cracks in the oxide film increased, and the fluctuation of the oxide film/matrix interface intensified. When dissolved oxygen content was relatively large, the mass increment of the alloy after corrosion was relatively large, and there were more cracks in the oxide film. After corrosion for 130 d in the environment containing dissolved oxygen, the self-corrosion potential and polarization resistance were the smallest, and the self-corrosion current density was the largest. The corrosion resistance of the alloy was not only related to the thickness of the oxide film on surface, but also affected by the defects in the oxide film. After corrosion for 300 d in the environment containing dissolved oxygen, the impedance spectrum showed the characteristics of multiple arc resistance. When the dissolved oxygen content was higher, the number of arc resistance was more.