The index of regional rainfall anomaly(I index) is defined as the regional average of representative station′s JJA standardized rainfall anomaly in South China,Changjiang-Huaihe valley,and North China.And it is proved that I index is consistent with commonly used Z and R index.The correlation of I between different regions is insignificant,indicating that the interannual changes of summer rainfall are independent in the above three regions.The I index is decomposed into the interdecadal variation index (I_s) and interannual variation index (I_f) by means of periodical analysis.And the variance analysis indicates that I_s is significant among the components of I only in South China,suggesting that the regional rainfall anomaly in summer is complicated.The analysis of the curves of I_s and I_f indicates that I_s in the three regions is related to the 1970s′ transition of Pacific Decadal Oscillation (PDO),and the significant negative anomaly of I_f in North China is contemporaneously close related to strong El Nino events.