According to the Meiyu data of mid-lower reaches of Changjiang River during 1885 to 2000 and the principal rainfall pattern during 1951 to 2000, Meiyu characteristics of the summer drought/flood pattern(SDFP) Ⅰ、Ⅱ、Ⅲ in China are statistically analyzed, and the causes of formation of the abnormal Meiyu and the abnormal drought/flooding after 1980s are studied. As a result, the Eastern Asian blocking high has been more frequently active since 1980s, which is unfavorable to the northward seasonal movement of west Pacific subtropical high. That is the reason why the Meiyu precipitation is rather rich in summer and the rainbelt is located on the southern China, but the further reason remains to be continued to study.