A severe rainfall process occurring in the east of Hubei province from 2000 BST of July 20 to 2000 BST of July 21,1998 was simulated by a η-coordinate mesoscale numerical model.According to the actural rainfall area, the humidity initial field at the level from 850hPa to 400hPa was adjusted.The test result shows that the forecast efficiency is strongly determined by the initial value of the mesoscale moisture at the mid-level and low-level.When the objective analysis on the radio-sounding data fails to correctly indicate the existence of the mesoscale high humidity area,the forecast extremely deviates from the actural situation.But when the humidity field is adjusted in accordance with the actual rainfall area,the forecast efficiency is greatly improved.The simulation effect reveals the importance of using intensive observation rainfall data to improve the initial field of the numerical model especially after the resolution of the model is increased.