Abstract: The biogeochemical processes in the methane-bearing surface sediments have been an interesting research field worldwide. Numerical simulation method is used in this study to quantify the biogeochemical processes in methane-bearing surface sediments of the Haiyang 4 Area, northern slope of the South China Sea. According to the actual geological data in the study area, a numerical transport-reaction model has been developed with Mathematica. According to the simulation results, it is inferred that there are methane sources in the study area. Methane could be rapidly transported to the surface sediments as gas bubbles from the methane reservoir in deep sediments. Gas irrigation could drive the exchange of solutes which led to the similarity of sulfate concentrations in the upper 0~2.8 m to those in the bottom water. The small inflow of methane from below (160 mmol·m^-2·a^-1) induces the low reactive rate of anaerobic methane oxidation (AOM) of 140 mmol·m^-2·a^-1 and a low carbonate precipitation rate of 35 mmol·m^-2·a^-1. This may be caused by a small or deep methane reservoir in the sediments. The well trap conditions can also prevent the escaping of methane. The precipitation rates of carbonate and sulfide turned out to be small in this area. The simulation also revealed that when the rate of sulfide mineral precipitation is high, the rate of carbonate precipitation will be diminished due to the alkalinity reduction.