Using a linear baroclinic model (LBM),the summer climatology basic flow is decomposed into barotropic and baroclinic components and reconstructed to study the role of different components of baroclinic basic flow in the low-frequency development and evolution of initial cyclone perturbation.The control run simulates several evolution characters similar to observations successfully,such as the north-westward propagation,the stall and enhancement in WNP(western-North Pacific)monsoon trough,and the formation of meridional wave train in East Asia and the northwest-southeast wave train form the South China Sea to the maritime continent.When the baroclinic component of basic flow is changed,the sensitive experiment shows that the barotropic flow cannot provide enough energy for the west-propagation of initial perturbation.The zonal temperature gradient and north wind vertical shear in East Asia due to ocean-land thermal contrast are the critical factor for the perturbation developing into EAP(East Asia Pacific)type merdional wave train and sustaining.When the zonal anomaly part of baroclinic flow increases linearly,the perturbation energy will intensify in the speed exponentially,which suggests in the climate change background,the tiny change of basic state may produce huge responses of weather or intraseasonal perturbation intensity.