Based on the 87 stations selected along middle and lower reaches of the Changjiang River, and high density surface meteorological precipitation daily data during 1957-2007, the statistical methods are used to analyze the rainstorm process， and establish a rainstorm process impact assessment model. First select average rainfall, rainfall intensity, coverage and duration as four indicators, each indicator uses the normal transformation or gamma distribution to fit, then uses the appropriate distribution’s inverse function of probability density function to determine the theoretical values of return period as the grade level standards， which can be divided into five levels. Finally, a composite assessment model of rainstorm process is set up. In this paper the model is applied to assessment of five rainstorm processes from the end of June to August in 1999, and the results show that the effect is good, and can be put into operation.