Various factors such as Dam discharge, algal photosynthesis, and sudden increase in water temperature in the natural environment can lead to supersaturation of Dissolved Oxygen (DO) or Total Dissolved Gas (TDG), which may cause fish to suffer from gas bubble disease or even death. In order to investigate the quantitative effect of algal photosynthesis alone on dissolved gas, especially TDG supersaturation, this paper carried out a quantitative study of the effect of algal photosynthesis on dissolved gas supersaturation in the Xiangjiaba reservoir area using the light-dark bottle method. Additionally, the relationship between dissolved gas saturation and algal density was analyzed in the downstream section of Jinsha River, considering the combined effect of dam release and algal photosynthesis and oxygen production. The results showed that: the increase of TDG saturation due to algal oxygen production ranged from 0.18% to 5.67%, significantly lower than the effect of the spillway building on TDG saturation. The effect of algal photosynthesis on dissolved gases was highly dependent on hydrodynamic factors. In tributaries, DO and TDG saturation exhibited a significant positive correlation with the algal density, whereas this relationship was negligible in the main channel. Algal photosynthesis had a more pronounced influence on DO saturation than on TDG saturation. The effect of algal photosynthesis on DO saturation was greater than that on TDG saturation, and the prototype observations at consecutive times showed that the variation of TDG saturation was much smaller than that of DO saturation. These findings provide a basis for clarifying the key drivers of dissolved gas supersaturation in reservoirs, and offer theoretical insights for mitigating the effects of dissolved gas supersaturation on fish.