Abstract: The particulate organic carbon content(POC)and particulate inorganic carbon content(PIC)in total suspended solid(TSS)are used to trace sediments diffusion bound in Bohai after the fourth test of water-sediment regulation of the Yellow River,the result indicates that POC and PIC in sediment are both stable with season changing,their mean content are(1.75×0.28)% and(0.51×0.08)% respectively.When TSS concentration is below 30 mg/L,PIC in TSS decrease swiftly to 0, whereas POC in TSS increase promptly.According to the analysis of chlorophyll-a and mineral in TSS,when TSS is less than 30 mg/L,phytoplankton increases,but the sediment from the Yellow River decreases in TSS.Simultaneously,ΣC₂₀^-/ΣC₂₀⁺ and carbon proponderance index(CPI)of n-alkane increase and decrease obviously.TSS=30 mg/L can be regarded as the effective boundary between the allochthonous TSS and autochthonous TSS.The sediment from the fourth test of water-sediment regulation of the Yellow River diffuses to southern river-mouth at surface layer;While at bottom layer,sediments diffusion range concentrates at northern and southern river-mouth of the Yellow River.the central section of Laizhou bay is likely to be the habitation of sediment from the fourth test of water-sediment regulation of the Yellow River.