By using the data of monthly mean temperature and humidity from 1971 to 2000 in Zhejiang Province, as well as the times and routes of migratory birds’ passing through Zhejiang Province, an analysis is made of the main periods of Avian Influenza (AI) prevailing in Zhejiang Province, according to the weather condition assessment standards such as temperature and humidity indexes presented by previous researchers, and the crucial periods with high risk for AI prevailing are identified. The results show that the climatic conditions from November of the current year to March of the next year are favorable to the surviving and spreading of AI in Zhejiang Province, especially in February and March. December and January is the middle risk period.