By using power spectum and bandpass filter,the quasi-periodic oscillation of Northern Hemisphere summer monsoon and Meiyu rainfall over the Changjiang-Huaihe river catchment in 1991 is investigated,with the monsoon's effect on the abnormal preeipitationexamined,baced on the exceptionally torrential rain data from May to August 1991.Resultsshow that there exists a noticeable quasi-24 day oscillation in the monsoon,humidity and precipitation fields。The oscillation found of the summer monsoon occurred chiefly in SW monsoon with its phase 2～4 days ahead of that of rainfall,and its amplitude indicating quite wellthe magnitude of the rainfall event.Hence,the quasiperiodic oscillation of the monsoon canbe utilized in Meiyu rainfall prediction for the river reaches。