By using the theory of moist potential vorticity(MPV,ξ(MPV)),a rainstorm happened in Shandong Province in April 2003 is analyzed.Results show that the rainstorm occurred in the neighbourhood of the stiff and dense section of the moist isentropes,where the convective stability was so small that it is beneficial to the development of moist baroclinic vorticity.The evolution of ξ(MPV1)<0 and ξ(MPV2)>0 at 700hPa level at the early stage of the rainstorm reflected convective instability and the intensification of baroclinic instability in the rainstorm area.The downward spread of high ξ(MPV)from the upper troposphere availed the release and storage of potential instability energy,resulting in the strengthening of precipitation.It is also an important mechanism through which the low vortex moved eastward and developed into a cyclone.