Abstract: With PSU/NCAR nonhydrostatic mesoscale model MM5, the rainfall process of typhoon Fitow(0114) is simulated during 00UTC31 Auguest-00UTC2 September 2001, Analyses on the model results show that MM5 model well reproduced the position and intensity of heavy rain. Mesoscale characters of heavy rain were quite distinct in rainfall time scale, rainfall area, stream field and divergence at lower and upper levels. the interaction between inverted Typhoon trough and these mesoscale systems leads to heavy rain happening. The distribution of divergence fields at lower and upper levels can have a kind of indication meaning for the heavy rain. The ageostrophic effect on heavy rain area were very strong at lower and upper levels. The ageostrophic vorticity forcing was an important factor for divergence tendency. The achieving of the ageostrophic effect on heavy rain in South China is distinct from the midhigh latitude area, The ageostrophic effect at lower levels was caused by the unbalance between strong geopotential field cyclonic vorticity((-?2 φ＜0) and weak stream field cyclonic vorticity(fζ＞0), The ageostrophic effect at upper levels was caused by the unbalance between strong geopotential field anticyclonic vorticity(-?2 φ＞0) and weak stream field anticyclonic vorticity(fζ＜0). The ageostrophic effect was trigger mechanism for ascending motion development in heavy rain mesoscale system. The cause that ageostrophic Q vector are better than quasi geostrophic Q vector divergence fields responding to the rainfall area was represented dynamically.