Abstract: Based on the FNL reanalysis data from NCEP, the preliminary stage of a heavy precipitation event caused by a southwest vortex in central eastern China during 25-26 May, 2013 is investigated by applying the potential vorticity theory.Results suggest that:there is a high potential vorticity anomaly near the plateau trough and favors the initial development of the low-layer of troposphere in the southwest vortex, it is quite an important implication for predicting the early stage of the southwest vortex. During the early stage of the southwest vortex, the barotropic term of the moist potential vorticity in the heavy rainfall center was in a "negative-positive" distribution from the lower to the upper layers, while the baroclinic term was in a large negative value center in the mid-lower layer, this re flects the enhancement effect of the low-layer convective instability and vertical wind shear on the convective instability. During the development stage of the southwest vortex, the positive value center of the barotropic term in the moist potential vorticity appeared a leaning funnel shape, and the baroclinic term in the low-layer of troposphere in the rainfall center occurred a large negative value center. The strong precipitation center has a good coincidence with the negative value of the disturbed moist potential vorticity in the low-layer of troposphere. These may, provide a reference for predicting the location of the heavy precipitation occurrence.