On the basis of the dynamic framework of the Weather Research and Forecasting Model (WRF) and the Morrison two-moment explicit cloud scheme, an AgI seeding scheme is developed and used to simulate stratiform precipitation during March 20-21, 2008. The simulated rainfall is similar to the observations, in which several seeding tests for different seeding amounts, seeding levels, and seeding times are examined on the basis of cloud microphysical analysis. The results show that seeding with an amount of 20 g slightly suppresses the surface rainfall in 30 min, rapidly enhances the surface rainfall after 30 min; the maximum suppressed and enhanced rainfall are 2010 t and 3.4×10⁵ t, respectively. The microphysical mechanism of decreasing rainfall is the depletion of supercooled water and an increase in snow and ice after AgI is injected into the cloud, which lead to the decrease conversion rate of rain collide with cloud water as same as cloud water auto converse into rain water but increase conversion rate of snow collide with rain, while enhanced snow cannot fall to the warm region to melt at this time. The microphysical mechanism that increased rainfall 30 min after the seeding operation is that enhanced snow gradually fell to the warm region and melted into rain. Different seeding amounts strongly influence the surface precipitation. Overseeding will reduce the average mass of snow crystals, causing the falling speed to sharply decline; thus, the amount of snow melting into rain is reduced, decreasing rainfall. Sensitive experiments with different seeding heights and seeding times show that when seeding is conducted in areas with a high supercooled water content and low ice particle content, the surface rainfall will be increased.