Abstract: Through the analysis of the data of dissolved oxygen from 2015 to 2019 in Mangjiu Reservoir, it was found that the annual data of dissolved oxygen was m-shaped. From the point of time, when the water temperature was low in winter, it happened to be the dry season of Mangjiu Reservoir, and the dissolved oxygen concentration was low. Further analysis of the relationship between dissolved oxygen (DO) and water temperature, chlorophyll content and transparency in Mangjiu Reservoir showed that the increase of water temperature would increase the solubility of oxygen in water, and the increase of water temperature and accelerate the reproduction of algae in water, the oxygen produced by photosynthesis of algae increased the concentration of dissolved oxygen in water, and the algae reproduction would reduce the transparency of water.