The analysis of observed salinity data over 35 years (1961-1996) at four stations around the Bohai Sea, i.e. Huludao, Qinhuangdao, Tanggu and Beihuangcheng, reveals that the salinity of the 4 observation stations has increased 1.1, 1.6, 1.9 and 0.4, respectively. The data also show that over the past 35 years, there have been at least 5 large salinity variation processes. The salinity data from two cruises of the Bohai Sea in August 1958 and 2000, show that the salinity pattern of the Bohai Sea has changed markedly. Low salinity in the sea surface layer around the old Yellow River mouth in August 1958 had been replaced by high salinity in August 2000 and the maximum variation of salinity is over 10.0. In addition, the values and distribution of salinity were almost the same from surface to bottom there in August 2000, but there existed significantly different salinity levels between the surface layer and the deep layer in August 1958. When a comparison is made between the salinity levels of the above-mentioned two years, it is found that the salinity in August 2000 is on average 2.0 higher than that of August 1958 in the main part of the Bohai Sea. The change of temperature and salinity field in the Bohai Sea leads to the change of the circulation. The numerical simulation shows that in comparison with the circulation structure of the Bohai Sea in August 1958, the circulation in August 2000 changes markedly. The significant changes of circulation appeared in Bohai Bay, Laizhou Bay and in the middle of the Bohai Sea. The clockwise current loop outside of the Bohai Bay and counterclockwise current loop outside of the Laizhou Bay in August 1958 disappeared in August 2000, and the counterclockwise current loop of the Bohai Bay migrated obviously outward. The flow direction in the Laizhou Bay turned 180° around. Corresponding to the variation of the Bohai Sea circulation, the amount of water exchange between the Bohai Sea and the Yellow Sea has also changed. The water exchange rate through the Bohai Strait decreases, on average, 0.7×104 m3/s in August 2000 in contrast to that of August 1958.