Zirconium alloys are important structural materials in pressurized water reactors. During actual operation, the corrosion resistance of water side is the most important factor affecting its service life. The oxide film of zirconium alloys formed during the corrosion process will reduce the heat transfer performance, mechanical properties and service life of the cladding material, thus becoming a factor restricting the development of nuclear power. The initial phase composition and the defect state in the crystal affect the microstructural evolution of the oxide film during the corrosion process, which in turn determines the late growth of the oxide film. In order to study the phase composition and crystal structure evolution of zirconium alloys from the initial oxidation to the formation of ZrO₂, the initial corrosion behavior of Zr-0.75Sn-0.35Fe-0.15Cr alloy was studied by using TEM thin foil specimens with coarse grains. The oxygen content varied due to the change of sample thickness at different distances along the perforation of TEM thin foil specimens with coarse grains, which could be investigated the crystal structure evolution of oxide film with the variation of oxygen content. Corrosion tests of these TEM specimens were conducted in an autoclave at 250 ℃ and 3 MPa in deionized water for short time exposure. The results showed a variation of the crystal structure along with the increase of oxygen contents at the initial oxidation stage. When the Zr/O atomic ratio reached 5~7, a commensurable long period super-lattice structure was formed. The lattice constants of the super-lattice (a, c) and α-Zr matrix (a₀, c₀) satisfied the relationship of a=9a₀ and c=2c₀, which was called 9a₀-2H structure. When the Zr/O atomic ratio reached 3 and 1, sub-oxides Zr₃O with hcp and ZrO with fcc ordered structures were formed, respectively. When the Zr/O atomic ratio was 0.85, monoclinic ZrO₂ was detected.