The EASWJ (East Asian Subtropical Westerly Jet) is one of the most important circulation systems in East Asia, which links the high and low latitude circulations over East Asia. The intraseasonal variation of position and intensity of EASWJ during June, July, August and the East Asian circulation characteristics as well as the physical processes are discussed by using statistical analysis and physical process diagnosis methods based on the NCEP/NCAR reanalysis data and NOAA ERSST V3 monthly mean sea surface temperature (SST) data from 1979 to 2008. The physical mechanism for intraseasonal shift of the EASWJ is explored. The study points out that in June, the position shift of the EASWJ is affected by the phase variation of eastward propagating Rossby waves in the Eurasian continent. The North Atlantic SST anomalies (colder or warmer than normal) in spring are the most significant forcing signal for the phase variation of Rossby waves in the Eurasian continent. In July, the position shift of the EASWJ is affected by the phase variation of Rossby waves propagating northward from the tropical to subtropical western Pacific. The tropical western Pacific SST anomalies in spring are the most significant forcing signal for the phase variation of Rossby waves over the western Pacific. In August, the position shift of the EASWJ is affected by the phase variation of Rossby waves propagating northward from South Asia to tropical and subtropical East Asia. The Indian Ocean SST anomalies in spring are the most significant forcing signal for the phase variation of Rossby wave propagating from South Asia to tropical and subtropical East Asia.