Abstract: The films of 2A12 aluminum alloy parts after anodizing were abnormally bright and had surface defects. The reasons for abnormal color and strip defects after anodizing of parts were studied by optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffraction. The results show that there was copper rich segregation phase and inclusion phase in the substrate of aluminum alloy. In the process of anodizing, the segregation phase in the substrate was preferentially dissolved, and the impurity phase was not oxidized, resulting in uneven anodic oxidation film, holes and inclusions in localized areas of film.