Abstract: By using daily precipitation data in Sichuan Province from 1961 to 2015, data during typical years on the rainstorm disaster, GDP, population and cultivated area, and the method of weighted summation based on normalized process precipitation, maximum daily precipitation and the duration, the comprehensive strength index and the grade standard of rainstorm are established. Based on the economic loss rate, population affected rate and crop affected rate indices, the grey correlation method is used to determine the disaster degree. The relational analysis method is used to establish the disaster loss assessment model based on the comprehensive strength index of rainstorm and the grey correlation degree of rainstorm, respectively. The possible economic loss rate caused by different return periods of rainstorm is studied, too. The result shows that the comprehensive strength index of rainstorm can reveal the time-frequency and spatial distribution characteristics of heavy rainfall in different intensities. The loss caused by rainstorm process is significantly related to the comprehensive strength index and the grey correlation degree of rainstorm, and the results of both the two kinds of loss assessment models are good. The economic loss rate of rainstorm of 5-year return period is mostly below 3%, and that of 50-year return period is above 3%. It in the north and west of Sichuan basin is more than 5%, and in some areas it could reach 7% to 9%.