Abstract: Based on the test results of the domestic tin Zr-4 C zirconium alloy in low alloy autoclave in the pure water and LiOH aqueous corrosion, using the transmission electron microscopy（TEM） observation matrix and oxide film microstructure, through the analysis of oxidation weight data, the corrosion mechanism of C zirconium alloy were studied. Three corrosion mechanisms are proposed: Nb can effectively inhibit the increasing of anion vacancy concentration and reduce the diffusion rate of the oxygen element. The number of deficient traps affects the corrosion caused by oxygen diffusion, and the number of empty traps is proportional to the total surface area of the second phase particles. The second phase of particle oxidation expansion leads to the relaxation of pressure stress of oxide film, which degrades its stability and protective ability.