Abstract: To overcome the deficiency of classic dicriminant method, the half polynomial was introduced in the discriminant,and the phase by phase linear discriminant method was developed. To test the new method, the May rainfall of Kunming and Mengzi was forecasted with the new method. The fitting accuracy of Kunming andMengzi are 85.7% and 82.9% for y^*＝y, but 77.1% and 71.4% for y^*＝y+ 40% y, and 80.0% and 74.3% but for y^*＝y-40% y respectively. The forecasting accuracyof Kunming and Mengzi are 6/7 and 6/7 for y^*＝y, 7/7 and 7/5 but for y^*＝y+40% y, and 5/7 and 7/7 but for y^*＝y-40% y respectively. The testingresults indicated that the phase by phase discriminant method had excellent fitting and forecasting ability.