One overpressured gas field and several gas -bearing traps were discovered recently in Yinggehai Basin, South China Sea, and the complexities of gas accumulation and distribution in overpressured domains(high pressure area and high pressure layer) is becoming more and more prominent. Based on the drilling, seismic, and test data, it is concluded that there are two kinds of sealing mechanism of mudstone caprock in the first member of the Upper Miocene Huangliu Fm. in X13 overpressured gas field, i.e., capillary sealing and overpressured sealing. The corresponding evaluation method of sealing capacity is setup, by which if the sum of caprock displacement pressure and overpressure exceeded the reservoir residual pressure, the mudstone had good sealing capacity. Combined with the burial history of the study area, the evolution process of sealing capacity of the mudstone caprock in the Huangliu Fm. is analyzed. The mudstone caprock began to have sealing capability at 6.1Ma. At 3.6Ma, the early charge period of natural gas, the mudstone caprock had capillary sealing capability with normal pressure. During 1.8~0Ma, the late charge period of natural gas, the mudstone caprock had both capillary sealing capability and overpressured sealing capability, in which the overpressure played a major role. The mudstone caprock had better sealing capability in the late charge period than in the early period. It is suggested that the traps(or plays)developed with an assemblage of such as overpressured layers in the middle of the mudstone, and compacted layers in the bottom of the mudstone, and some large-scale reservoirs, would be the favorable targets in overpressured domains.