Based on the current standard and method to classify the tenday rainfall of Jilin Province at present, a new method, which is based on the pro bability distribution function, is presented. In view of the nonnormal distrib u tion of tenday rainfall, Z index transformation is employed. The index can tran sform the nonnormal distribution data into the normal one. The classification r esult of rainfall is based on classifying the Z index. The results of the n ew method are more stable and more rational than those of the original method.