Abstract: The spatio-temporal variations of the internal dynamic structure induced by eddy evolution are the basis for the ecological dynamic study of eddy. Using the high-resolution reanalysis data of temperature, salinity and velocity in the Kuroshio-Oyashio Confluence Region, the three-dimensional eddy detection method, normalized composite analysis, and the maximum angle method are used to detect eddies, construct the three-dimensional structure of eddy, and calculate the thermocline and pycnocline parameters within eddies. Based on the statistical analysis of the eddy dynamic parameters, the eddy lifetime is divided into three different evolution periods: development, stability and decay. In this study, the 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 the thermocline is also discussed. The results show that there are more cyclonic than anticyclonic eddies in the Kuroshio-Oyashio Confluence Region. However, the anticyclonic eddies have larger mean radius and longer lifetimes, and their influence depths are also deeper than that of the cyclonic eddy. In other words, the anticyclonic eddy is stronger and more stable than cyclonic eddy in the Kuroshio-Oyashio Confluence Region. The cold core inside the cyclonic eddy or the warm core in the anticyclonic eddy both increase during the eddy development period and then weaken from stable to decay period. Comparing with the cyclonic eddy, the fluctuation range of the anticyclonic eddy is larger. Both cyclonic and anticyclonic eddies, the anomaly in internal salinity is bounded by 400 meters and exhibits an opposite horizontal distribution. The upper layer of the anticyclonic eddy is high inside and low outside, and the deeper layer is low inside and high outside. The cyclonic eddy has opposite characters. Both eddy types are tilted westward with the most significant performance at the stability period. Due to the combined effects of eddy mixing and pump, the thermocline depth inside the eddy gradually deepens in the development period, becomes shallow in the stable period, and then increases slightly in the decay period. The thermocline intensity tends to decrease gradually with the evolution of eddy. The anticyclonic eddies are more obvious than cyclonic eddies.