Spore releasing regulation and infection period of Ustilaginoidea virens were carried out by using spore catcher and randomized block design in 2008-2009, and the optimum period of controlling rice false smut in the single-middle-season rice area in Anhui Province was also studied. The results showed that spores appeared in around mid-July, and formed the first release peak in late July to early August. Spores releasing in this stage was the primary source of infection. After the appearance of symptom in late August, spores increased rapidly. The release was mainly at night (1:00-5:00 am) in sunny day, while in cloudy-rainy day, it would last all day long without a distinct peak. The incidence of rice was different when inoculated with hypha and spore suspension of U. virens in different growth periods. Inoculated at the exposing of flag leaf apex stage, rice was not infected; from flag-leaf-deployment to flowering, the incidence was from 1.2％ to 76.4％; in booting stage(5 days after the entirely expanding of flag leaves), the incidence was the highest. The control effects of fungicides spraying at flag-leaf-deployment stage were all over 78.5%, better than that in advance or delay. Therefore, the optimal time for controlling rice false smut was at flag-leaf-deployment stage.