The stability and reliability for drought/flood trend prediction from the statistical model are depended on the selection and synthesis of the predictors. While selecting predictors, the physical significance of the predictors must be considered. According to the scale correspondence principle, the synthesis of the factors whose time scales correspond with those of the predictands can retain the useful information to the utmost extent. A comparison experiment is conducted for the precipitation field over the Changjiang River Delta in flood season and the result indicates that the method can improve forecast greatly.