Based on the data of temperature,salinity and DO observed in the Changjiang Estuary covering 54 stations in late summer,2009,vertical structure types of water temperature were classified by the method of vertical gradient,and the parameters including depth and intensity of thermocline and vertical gradient of DO on the water layer of thermocline were calculated.In late summer,the distribution of temperature vertical structure and intensity of thermocline off the Changjiang Estuary was a situation of confluence and mixing by the dilute water of Changjiang,the coastal current of Yellow Sea,the warm upper water of Taiwan Warm Current and the cold deep water of Taiwan Warm Current.Correlation analysis showed that,there was a negative correlation between the depth of thermocline and the vertical gradient of DO on the layer of thermocline,there was a positive correlation between the thermocline depth and the vertical minimum DO,and also a positive correlation between the thermocline intensity and vertical gradient of DO on the layer of thermocline.The thermocline intensity,especially the intensity of upper thermocline which was near sea surface,the origin of DO,has a strong correlation(r=0.69)with the DO vertical gradient on the layer of thermocline.The shallower and the stronger of the thermocline,the greater effect of the thermocline on the hypoxia of deep water.Thermocline,in particular,the upper thermocline hindered the surface water with saturated DO mixed with the deep water,played a role of physical barrier of oxygen vertical transportation.The thermocline was an important environmental control factor of the hypoxia off the Changjiang Estuary from late spring to autumn,covering a period of occurrence,development,maintenance and decay of the hypoxia.