Abstract: Using the two CTD data of the summertime in 2000 and wintertime in 2001 in the Bohai Sea,the temperature,salinity and density distributional characteristics of the wintertime and summer time in the Bohai Sea are analysed systemically and it is revealed that the current hydrologic characteristics of the Bohai Sea have some new phenomena compared with the previous research results.In summer there are three low temperature centers in the middle layer and underlayer of the middle parts of the Bohai Straits,the Liaodong Gulf mouth and the Bohai Gulf mouth but there is a uniform high temperature center in the middle Bohai Sea.In winter the isotherms extend westward and northward to the Bohai Sea from the warm core of the Bohai Straits and the cold water tongue off the Qinhuangdao spreads southeastward out,so the saddle like isotherm pattern is formed in the middle Bohai Sea.The salinity in the Bohai Sea in summer and winter rises significantly compared with the previous results and is higher than the northwestern Huanghai Sea.The salinity distribution characteristics are the high salinity in the Bohai Bay and the low salinity in the Bohai Straits.The density distribution shows that in summer there is a large cyclonic density circulation in the Bohai Sea and it is likely to exist some density current eddies in the middle part.In winter the saddle like pattern of the Bohai Sea temperature and salinity shows that the circulation structure is much complexer than the previous research results.The Huanghai Sea Warm Current(extension)has branches in the Bohai Sea.The model diagnostic results of the thermohaline residual currents accord well with the observed data.