Abstract: In this paper, the phenomenon of early Mei-yu over the Jiang-huai area in 1991 and its formation mechanism have been studied. The results indicate that in the fourth pentad in May, the first northward-jumping of the west Pacific subtropical ridge occurred, and the area south of the Yangtze river valley started to be influenced by the summer monsoon. and the onset of the Mei-yu period over the Jiang-huai area was more than twenty days earlier than that of the normal years; these are related to the relatively strong cross equatorial air current which appeared abnormally between 120°E and 130°E from the second pentad to the fifth pentad in May.