To explore the causes of early-spring diatom bloom in the middle and lower reaches of Hanjiang River, we took unamended water sample from Hanjiang River before the diatom bloom in the early spring of 2010 to study the biological characteristics of Stephanodiscus hantzschii in terms of water velocity, illumination intensity and water temperature. The results indicated that Stephanodiscus hantzschii adapted well to water velocity of 0-0.5 m/s, and could grow well in the water temperature of 2-15℃, especially in 2℃. There was no significant difference in the diatom growth abundance when the illumination intensity is of 15 and 25 μE/(m2·s). It’s concluded that low water velocity and suitable climate are main incentives of diatom bloom outbreak in early spring. It’s also pointed out that the eutrophication is becoming serious as  nutrients concentration in the Hanjiang River could meet with the demand of water bloom outbreak.