In view of the fact that the practical salinity of the Bohai Sea in January 2007 was significantly higher than the multi-year mean with its horizontal distribution opposite to the latter, the factors that affect the interannual variation of practical salinity in the Bohai Sea are quantitatively analyzed based on the in-situ hydrological measurements, the annual runoff data of the Yellow River into the sea, as well as the precipitation and evaporation reanalysis data of EAR5. The results show that the local freshwater supply not only dominates the magnitude of salinity change in the Bohai Sea, but also causes the salinity in the central Bohai Sea is higher than that in the Bohai Strait in winter in some years which is in inverse to the climatological salinity field. The seesaw distribution characteristics of the freshwater supply of the Bohai, Yellow and East China Seas also strengthen the characteristics that the salinity horizontal distribution opposite to the climatological in the Bohai Sea in some years. Available observations also show that the nutrient and inorganic carbon of the Bohai Sea are much higher than that of the open ocean, which gives a rise of 0.02 ~0.2 g·kg-1 in Absolute Salinity. Therefore, it is necessary to replace the Practical Salinity with the Absolute Salinity for the accurately salinity changes study in the Bohai Sea.