Abstract: The relationships between the precipitation over Eastern Asia (20°-45°N, 110°-135°E) and the 30-60 day intraseasonal oscillations (ISO) over the boreal Pacific during the summer are studied. The daily wind and height fields of NCEP/NCAR reanalysis data and the 24 h precipitation data of China 687 stations during 1958-2000, and the pentad precipitation of CMAP/NOAA from 1979 to 2002, are all analyzed by the Space-time filter analysis. The analysis results, from every drought and flood summer for 4 different regions of East Asia respectively during 1958-2000, showed that the precipitation amount in the East Asian summer monsoon region is well correlated to the westward propagation of 30-60 day ISO via the north central eastern Pacific, and depends little on the intensity changes of the East Asian summer monsoon. And the westward ISO is usually the low-frequency low and high pressure systems from the Bay of Alaska in northeastern Pacific and the Okhotsk in the northwestern Pacific of mid high latitudes, and the ISO evolving in subtropical easterlies. In mid high latitude the phenomena are related to the westward propagating mid ocean trough and block high backing off. There fore the westward propagating ISO from the north central-eastern Pacificto East Asia is indispensable to sufficient rainfall occurring for East Asia in summer,which is resulted from long wave adjustment process in the mid-high latitudes and ISO evolving in tropical easterlies.