Using the 6 stations precipitation data in the Sanxia area and weather chart data from October to January during 1961—1994, the prior indexes of regional severe precipitation in Sanxia area are analysed under 500hPa circulation of "big U"pattern. It is tested using the data in 1995 and trial forcasted using the data in 1996. The Result shows that according to the combination of indexes including 500hPa small trough-line, trough-line over the Bay of Bengal and frontal-line on surface chart, added to some other conditions,it may be forecasted if there is regional severe Precipitation under the 500hPa circulation of "big U"pattern.