Abstract:By using the helicity and moist potential vorticity (MPV) theory 1° ×1° reanal ysis data of NCEP was calculated to diagnose the heavy rain process of NO.18 Typ hoon AERE in the east of Henan. The dynamical and thermodynamic mechanisms of t his process' formation and development were discussed. The results show that so utheast jet in the typhoon inverted trough periphery supplied vapor and thermody namic conditions for the heavy rain. Strong convergence action on the top of typ hoon inverted trough was the dynamical mechanism of the rainstorm' happening and developing. Because of weak cool air intrusion in the low level the MPV1 o f ra instorm area turned from negative to positive, which led vertical vortexes to en hance and hasten precipitation developing. So the weak cool air intrusion increa sed the precipitation in some extent. Moreover, the moving and developing of big value band of the vertical helicity of 925hPa could dictate the place of invert ed trough and had better indicative meaning for rainstorm falling area. When the rainstorm happened the area was in the negative big value area of MPV1.