A case of heavy rainfall during the Meiyu period of 22－23 July 2002 is simulated using mesoscale model MM5 coupled with the CAMS (Chinese Academy of Meteorological Sciences) microphysical scheme. Numerical experiments of adding ice crystals are conducted to study if there is any possibility to decrease the heavy rainfall. Several tests for different seeding times and different concentrations of seeding particles are designed. Results show that overseeding in the mature stage results in the most reduction of rainfall. In the region of 3600 km2, rainfall is decreased by 14.8%, which is 8.29×106 t. The heavy rainfall area with precipitation over 50 mm decreases from 190 km2 to 60 km2. After seeding with a large amount of ice crystals, graupel grows through the collection of ice crystal with supercooled rain water. This causes that the number concentration of graupel becomes larger and the mean diameter of graupel becomes smaller. The falling speed of graupel becomes smaller than the updraft and graupel cannot fall to the warm region to melt. This is one of the main reasons of decreasing surface rainfall. While in the weak updraft region around the decreasing rainfall center, graupel retained in the cold area grows gradually, and then falls, which increases the rainfall in the weak precipitation region. The overseeding methods in this paper are able to be used technically in the practical field test, which is worthy to make further studies and tests.