By using the precipitation data in the dry season from 1961 to 1994 and the same time synoptic map, the severe precipitation factors and features of circulation prior to the occurrence of severe precipitation are analysed. The two trough-one ridge pattern is classified from numerous facts and the indexes of forcasting severe precipitation are discovered from the daily data. The forecasting indexes of the severe precipitation for the two trough-one ridge pattern are given,it is proved to be practical value in the dry season in Sanxia on the Changjiang in recent two years.