A nychthemeron monitoring was performed around the Potamogeton crispus population in order to study the diurnal variation of water quality around Potamogeton crispus population. Result showed that, within the waters containing Potamogeton crispus population, the dissolved oxygen (DO) concentration increased from sunrise and droped continuously after sunset, following the water temperature variation. DO declined particularly apparently in the evening around 19:30, experienced a short-term recovery and then decreased until sunrise. The DO concentration within the open waters significantly correlated with water temperature variation; the DO variation tendency in the the boundary waters had the characters of that both for population-contained and open waters.The pH value within the population area increased after sunrise, and reached the highest at sunset, and then declined until sunrise on the following day, which was similar with that in the boundary waters; the pH values in the open waters area showed unobvious diurnal tendency. The DO concentration and pH value distribution followed this pattern: population area > boundary area > open waters. The total nitrogen (TN) content in the population and boundary area was highest around the sunrise and then decreased gradually, while the TN content at the open waters in the night was higher than in the day, and the highest TN content appeared at sunrise. The NH₄⁺-N content variation showed no significant diurnal changes. The total phosphorus (TP) content with the population-contained area was more in the evening than in the day. The TP content reached the highest value at the sunrise, dropped rapidly, and then increased after sunset. The TP content in the open waters is higher in the evening than that in the day, with the lowest value appearing at the midday and highest value at dusk. Both TN and TP showed following distribution patterns: population area < boundary area < open waters. Potamogeton crispus had an important impact on DO, pH, and other water environmental factors, which in turn affected the migration of the endogenous nitrogen and phosphorus. Potamogeton crispus population had no significant effect on both pH value and the migration of nitrogen and phosphorus. Moreover, diurnal variation of DO had greater impact on the migration of endogenous nitrogen and phosphorus compared to photosynthesis.