A rainstorm that caused a severe flood along the Yangtze River Valley in summer 1998 is investigated on the basis of weather maps and other data.The result shows that the occurrence and development of the flood-causing rainstorm is triggered by frequent activitives of mesoscale systems in the middle- and low- atmosphere under certain circulation pattern.Two circulation patterns which cause the rainstorm and lead to the flood peak in the valley are put forward,namely:pattern of strengthening subtropical high and eastward-moving trough;pattern of stable subtropical high and frequent southwest eddy activitives on shear line.It is concluded that the more(less) frequent of the appearance of the two patterns and rainstorm, the more likely the formation of flood(draught) in the valley.