The response of water quality to different growth periods of Potamogeton crispus was studied by in situ observation. The results show that the pH and ORP values and mass concentration of DO increase and the mass concentration of COD decreases during exponential growth period and stone bud formation period. On the contrary, in the period of plant decay, the mass concentration of TP increases due to the decay of Potamogeton crispus, but the mass concentration of TN and nitrate nitrogen decreases due to denitrification. In the process of growth, Potamogeton crispus absorbs nitrate nitrogen directly, but removes ammonia nitrogen indirectly. At this time, nitrate nitrogen is the main nitrogen in water. Ammonia nitrogen becomes the main form of nitrogen in water when Potamogeton crispus decay. The density of Potamogeton crispus is an important factor affecting the concentration of different nitrogen forms in water. The density of Potamogeton crispus is negatively correlated with the concentration of TN and nitrate nitrogen(P < 0. 01)during the growth of Potamogeton crispus. When the Potamogeton crispus decays, the mass concentration of TN and nitrate nitrogen in the Potamogeton crispus intensive area is lower than that in the sparse area, while the mass concentration of ammonia nitrogen was higher. During the exponential growth period, the optimum density of Potamogeton crispus in Luoma Lake is 83.75g/m³.