Abstract: By using the concept of potential vorticity, the characteristics of potential vorticity and other corresponding physical quantities are analyzed in the process of excessively heavy rain over the Changjiang-Huaihe River basin in 1991. It is shown that the strong precipitation area is located in the lower value area of dry potential vorticity and at the warm side of negative wet potential vorticity center, and it is related directively to relative wet potential vorticity. The baroclinic part of wet potential vorticity can reflect clearly the role of wet baroclinity in the convectively instable systems. There is a stronger response to frontgenetical forcing in the atmosphere where the value of wet potential vorticity is relatively small.