Through indoor experiments simulating the decomposition process of Potamogeton crispus, the decomposition law of Potamogeton crispus and its impact on water quality were studied. The results showed that: (a) the residual dry matter rate during the decomposition process of Potamogeton crispus showed an exponential trend, with a residual dry matter rate of 30–40% at 44 days; (b) The decomposition process of Potamogeton crispus releases nutrients such as C, N, and P into the water, with a release rate of C > P > N; (c) The decomposition of Potamogeton crispus is significantly affected by microbial action; (d) The decomposition of Potamogeton crispus increases the concentration of nutrients in the water, which has a certain impact on the water quality of Baiyangdian and poses a risk of eutrophication. It is recommended to take measures such as harvesting during the vigorous growth period of the Potamogeton crispus and timely salvage after its decline to prevent the risk of water quality deterioration caused by the decomposition of Potamogeton crispus.