Abstract: The basic reasons of algalbloom occurrence in middle and lower reaches of Hanjiang River are terrible pollution of water in lower reaches,sun light and rising water temperature in spring,and the slow velocity of flow in Hanjiang River as the result of low water in Hanjiang River as well as relatively high water level in Yangtze River.A hydrodynamics model and eutrophication model are established,providing pollution load maintains the status quo.By sampling in each aggregate of environmental factor which conduces toal-galbloom,the probability of algalbloom occurrence around transfering water are calculated by using the models.The result indicates that the probability will be increased after transferring 14.15 billion m³ water from Hanjiang River,but decreased after finishing of the water transfer project from Yangtze River to Hanjiang River.