Abstract: A pentad East Asia Pacific(EAP) pattern index based on 850 hPa tonal wind anomalies is established to analyze the intrascasonal EAP variability.It is shown that the EAP-like meridional wave train is the dominant mode of the East Asian Summer Monsoon(EASM) intrascasonal variability.The evolution process is as follows; disturbances firstly occur in the middle of the North Pacific, barotropic energy conversion from the middle latitude mean flow facilitates the development of the middle latitudc center of the anomaly circulation,and the southward dispersion of the Rossby wave energy excites convection anomaly and equatorial Rossby waves which tend to reinforce each other over the tropical western Pacific region to form the main EAP pattern.The low and mid-latitude anomalies then locked together and both move westward because of the equatorial Rossby-wave beta-effect control on its tropical center.The anomalous convection and circulation become strongest near Philippines when a tripolar wave train appears in the East Asian coast, and then the low and mid-latitude anomalies continue moving northwest-ward and weakened though in the Yangtze valley basin precipitation anomaly is able to maintain.The reasons why the EAP pattern develops and maintains in East Asian are as follows:first of all,there exists the positive feedback between convection over the Philippines sea warm pool and the low-level circulation;secondly, the EAP pattern tilts to the north in the meridional direction,resulting from a significant energy supply from baroclinic energy conversion from mean available potential energy associated with land-ocean tonal temperature contrast and thus the meridional wind shear between high and low levels.It also draws energy from mean available potential energy owing to the meridional temperature gradient.