The outer gill of female Hyriopsis cumingii is not only a respiratory organ, but also performs the function of embryo sac during the reproduction period. The respiratory efficiency of female Hyriopsis cumingii was decreased during the breeding period. The annual changes of the outer gill tissue morphology of female Hyriopsis cumingii were observed by paraffin section, and the effects of hypoxia environment on the outer gill tissue morphology and enzyme activities of female Hyriopsis cumingii were explored. The results showed that the outer gill of female Hyriopsis cumingii was the thinnest in January, and increased to the thickest in May, which was 173.86% thicker than that in January. The gill gradually became thinner, and the width of the gill cavity also increased significantly with the thickness of the gill, and increased to the widest in June, which was 171.22% wider than that in January, and the gill cavity gradually narrowed there were a large number of early embryos. In hypoxia environment, the activity of lactate dehydrogenase (LDH) increased gradually in 1-5 days, reached the maximum value on the 5th day, increased by 131.1% compared with the control group, and decreased slowly in 5-9 days; The activity of superoxide dismutase (SOD) decreased gradually in 1-7 days, reached the minimum value on the 7th day, decreased by 40.53% compared with the control group, and increased slowly in 7-9 days; The activity of catalase (CAT) increased gradually in 1-7 days. The activity of succinate dehydrogenase (SDH) decreased from 1 to 9 days, and the lowest value appeared on the 9th day, which was 68.9% lower than that of the control group. After 5 days of hypoxia stress, the cilia on the lateral side of the outer gill were 79.58% wider than that of the control group, and the aqueduct of the outer gill were 248.73% wider than that of the control group. This study provided basic data for the changes of outer gill tissue structure of female Hyriopsis cumingii and the effects of low dissolved oxygen on the tissue structure and enzyme activities of outer gill of female Hyriopsis cumingii, and provided theoretical basis for improving the breeding technology of Hyriopsis cumingii.