A heavy rainfall event occurred over the south coast of Zhejiang Province from 29 September to 1 October 2009, caused by an easterly wave, is studied with the conventional meteorological data, mesoscale automatic weather station data, and 1°×1° reanalyzed NCEP data. The results show that the outer circumfluence of the Typhoon Ketsana provided disturbance for the easterly wave; the lowerlevel southeast jet, induced by the enhancement of the subtropical anticyclone over the western Pacific, transported a plenty of moisture for the storm; the weak cool air strengthened the precipitation. The diagnostic analysis of physical quantities based on the NCEP reanalysis data indicates that the easterly wave happened at lower levels and caused a mesoscale low vortex in the latter period. The vorticity and helicity at 850 hPa can reflect the evolution of the easterly wave and the mesoscale low vortex perfectly, as well as the areas of strong precipitation. The moisture flux divergence at 850 hPa indicates that the southeast jet provided enough moisture for the storm.