Abstract: An divisional barptropic and baroclinic connection model is employed to study the South China Sea(SCS) upper circulation characteritics and its dynamic mechanism in summer.The results are:during summer,the strength and position of the cyclonic eddy adjacent to Dongsha Islands varies due to the instability of wind-driven current,which indirectly cause the intrusion extent of Kuroshio into the northern SCS,the strength and range of the anticyclonicgyre to the south of the above eddy,the cyclonic eddy to the south west of Xisha Isalnds etc.to fluctuate.The northward western boundary current in the southern SCS flows northeastw ard off the shoreline of middle Indo-China Peninsular due to the driving of southwest monsoon offshore,which brings about loss of lar ge amount water alongshore and the for mation of a southw ard compensating current alongshore,and then a cyclonic eddy to the southwest of Xisha Isalnds is induced.The above offshore western boundary current flows clockwise,thus forms an anticyclonic gyre in the southern SCS.The local cyclonic eddies adjacent to Nansha Trough and Wanan Shoals are formed by the actions of effect and bottom to pographic effect.