Abstract: A 3-D baroclinic ocean model is used to simulate the seasonal variation of temperature in Bohai Sea.Based on the difference(△T) of sea surface temperature(SST) and sea bottom temperature(SBT),it is found that △T is less than 0.5℃ throughout Bohai Sea before March,meaning that water column is mixed sufficiently; when April comes,the over 2℃ temperature difference occurs in Laizhou Bay,central basin and Bohai Sea strait,implying that the vertical stratification comes into being; the tendency that stratification area increases with heat flux through sea surface can extend to August.Thereafter,with the decreased irradiation and strengthened tidal mixing as well as wind stirring,the seasonal thermocline disappears gradually,and the water column resumes the fully mixed state.