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By means of monthly rainfall and temperature data from 1953 to 2006 at seven stations in Shiyan， the summer droughts are divided by the traditional method (R index). Considering that besides rainfall, temperature has also important effect on droughts, P index is designed to divide summer droughts. The results indicate that Shiyan is one of areas attacked by droughts frequently; there exists periodicity and continuity in Shiyan droughts; droughts are severer in the northern part than those in the southern part. The main influencing weather systems include the East Asia trough, subtropical high, QinghaiTibet high (mainland high), which influence Shiyan alternately,controlled by East Asia trough in spring, by the subtropical high and mainland high in summer, and by the mainland high and subtropical high in autumn. Detailed forecast indexes are given according to different seasons and drought types.