Abstract: The spatio-temporal variation of the internal dynamic structure associated with eddy evolution is critical important to the study of ecological dynamics. Based on high-resolution reanalysis data of temperature, salinity and velocity in the Kuroshio-Oyashio Confluence Region (KOCR), the three-dimensional eddy detection method, normalized composite analysis, and the maximum angle method are used to detect the eddies, construct the three-dimensional structure of eddy, and calculate the thermocline and pycnocline parameters of eddies. With the statistical analysis of eddy dynamic parameters, eddy lifetime is divided into three different evolution periods: development, stability, and decay. In this study, three-dimensional eddy structures of density, temperature, and salinity are systematically analyzed at various depths within three times radius according to different eddy types. The influence of eddy movement on the depth and intensity of thermocline is also discussed. The results show that more cyclonic than anticyclonic eddies exist in the KOCR. However, the anticyclonic eddies have larger mean radius and longer lifetime, and their influence depth is deeper than that of the cyclonic eddies. In other words, the anticyclonic eddies are stronger and more stable than cyclonic eddies. The cold core inside a cyclonic eddy or the warm core in a anticyclonic eddy both increase during the eddy development period and then weaken from stable to decay period. Comparing with cyclonic eddies, the fluctuation range of anticyclonic eddies is larger. For both cyclonic and anticyclonic eddies, the salinity anomaly inside the eddy is bounded by 400 meters and exhibits an opposite horizontal distribution: the salinity anomaly in upper layer of the anticyclonic eddy is high inside and low outside, while in deeper layer it is low inside and high outside, and the cyclonic eddy has opposite characters. Both types of eddies tilte westward and are most significant in the stability period. Due to combined effect of eddy mixing and pumping, the thermocline depth inside eddies gradually deepens in the development period, and becomes shallow in the stable period, then increases slightly in the decay period. The thermocline intensity tends to decrease gradually as eddies evolute, and this phenomenon is more obvious for cyclonic eddies.