Abstract: The torrential rain process occurred again in the lower and middle reaches of Yangtze River on 21-22 July 1998,due to the fact that the subtropical high decreased and with-drawn southward. By using of cloud picture,an analysis of the characteristics of weather system which caused torrential rain process was made and it is shown that,fristly,the torrential rain was caused by two meso-β scale cloud clusters which appeared in cumulus area between meso-α scale cloud clusters and its life cycle was about 10 h;secondly,the cloud clusters occurred in the area with rich vapor and energy and over the 850-700 hPa shear line with strong ageostrophic wind,and was located in the moist side of cloud system boundary.