Abstract: Large amount of shallow biogenic gas occurs in the marine sediments of Hangzhou Bay. As an important greenhouse gas and carbon carrier, methane and its anaerobic oxidation (AOM) and carbon cycle within the sulfate-methane transition zone (SMTZ) in marine sediments are of great significance for accurately assessment of the eco-environmental effects. Based on the test results and geochemical parameters, such as those from pore water and headspace gas in the YS6 sediment cores, following the principles of mass conservation and carbon isotope mass conservation, the internal carbon cycle in SMTZ for the YS6 was quantitatively studied with the “box model”. It is found that the SMTZ occurs in the 6～8 mbsf sediment layer.In addition to organoclastic sulfate reduction (OSR), AOM and carbonate precipitation (CP), there are concealed methanogenesis by carbon dioxide reduction of DIC produced from AOM (CR). However, methanogenesis from organic matter degradation (ME) almost not observed in the SMTZ-internal carbon cycling. The reaction rates of OSR, AOM, CP, CR and ME were 9.14 mmol·m⁻²·yr⁻¹, 7.42 mmol·m⁻²·yr⁻¹, 4.36 mmol·m⁻²·yr⁻¹, 2.72 mmol·m⁻²·yr⁻¹ and 0.00 mmol·m⁻²·yr⁻¹, respectively. The contribution rate of each reaction to pore water DIC in SMTZ was in an order of OSR＞AOM＞ME (ME= 0), while the consumption rate was CP＞CR. Methane diffused upward from deeper methane zone was not the only electron donor to drive the internal sulfate reduction (SR) in SMTZ. CR and OSR were also the important factors for sulfate flux into SMTZ to be greater than methan , and the obvious ¹³C-depletion of methane in the lower border of SMTZ was also related to the CR. When quantitatively evaluating the relative strength of AOM in marine sediments, the “cryptic” methanogenesis (such as CR, ME, etc.) in SMTZ cannot be ignored.