Water quality prediction is the basis of water environmental planning, evaluation and management. A gray dynamic model group is put forward made up of six simple gray models. Then the model group is used to predict the trend of concentration of NH₃-N in the Huaihe river during dry season. The result shows that the gray dynamic model group can make full use of the recent information about water quality to predict the future trend of water quality, and that prediction result stemming from gray dynamic model group is more accurate and reliable than that of a simple gray model