In this paper, salinity field is numerically simulated by using unstructured grid FVCOM (finite volume coastal ocean model) to analyze the seasonal characteristics of salinity in Bohai Sea and the influence of Yellow River runoff and precipitation on salinity is also studied through sensitivity experiment. The results show that the vertical distribution of salinity in Bohai Sea is homogeneous and seasonal characteristics of salinity are that it increases in winter and gradually decreases from autumn to summer, but increases again in autumn and gradually restores to the winter level. The Yellow River runoff and precipitation can find their influence mainly in summer and autumn. The influence of Yellow River runoff concentrates in the area nearby Laizhou Bay, which results in change rate of salinity up to 20%, while the influence of precipitation sweeps through almost the entire Bohai Sea, which makes the change rate of salinity reach its maximum for the whole year in summer, that is 9% for Bohai Gulf, 7% for Liaodong Gulf and 6% for Laizhou Bay.