Abstract: The vertical distribution of sulfate， free methane， dissolved inorganic carbon and δ¹³C_DIC in pore water of the sediments in the Pearl River Estuary was analyzed used methods of pore water chemistry and stable isotope chemistry. The results showed that concentration of pore water sulfate decreased linearly from the top to the bottom. Near the sulfate and methane interface， almost all sulfates were consumed and the concentration of methane increased sharply. The sulfate and methane interface of KC1， KC3， KC4， K17 and K10 were at 20 cm， 50 cm， 70 cm， 80 cm and 50 cm， respectively. The depth of SMI increased gradually from the mangrove area to the bare flat. At the same time， the concentration of DIC increased significantly in the pore water. The intensive methane anaerobic oxidation occurred in the sediments. During the AOM process， due to faster ¹²CH₄ oxidation rate than ¹³CH₄， it lighted δ¹³C_DIC in sediments pore water. The results showed that the content of organic matter and its activity in the sediments were the key factors controlling the depth of SMI. High content of active organic matter could accelerate the remineralization process of pore water sulfate and increased correspondingly the sulfate flux through AOM. Under the action of microorganisms， parts of labile organic matter （mainly algaderived） were consumed and the amount of organic matter entering the base of sulfate reduction zone reduced， which promote the AOM action due to reactions of partial sulfate with methane.