This paper studied the effects of water hyacinth in improving the water quality. Based on the analysis of the comparison of the water quality determination, this paper shows that under the average water temperature of 15 ℃ and average illuminate of 1 379 Lx, 1 kg water hyacinth absorbs PＯ４３－- P 5. 0 mg/d and NO2 - N and NH4+ - N 47. 9 mg/d in average. In 15 days water hyacinth eliminates the NO2- - N from the water in the pond. These indicate that the water hyacinth is very effective in the improvement of the water quality and the reduction of the water pollution.