Pore-water pressure sealing mechanism and hydrocarbon concentration sealing mechanism have been proved to exist in the shallow biogenic gas cap beds of Late Quaternary in Hangzhou Bay Area. At present, the dispute over the existance  of capillary sealing mechanism still exists. The cap beds of shallow biogenic gas reservoirs of Late Quaternary in Hangzhou Bay Area are shallow buried, with the characteristics of new age and loose construction. Therefore, we can’t apply mechanically the evaluation parameters and methods of deep diagenetic cap beds to evaluating capillary sealing mechanism in the study area. In this study, we measure the permeabilities of samples from cap beds and the reservoirs by falling-head permeability test, and we set the permeability as the evaluation parameter on the capillary sealing capacity of the shallow biogenic gas cap beds in order to study the characteristics and influencing factors of capillary sealing mechanism in shallow biogenic gas cap beds of the study area in detail. The difference of three orders of magnitude in permeability measured by falling-head permeability test between cap beds and reservoirs shows that there has been strong capillary sealing ability in cap beds, which leads  to the formation of shallow biogenic gas pools. The capillary sealing mechanism of shallow biogenic cap beds are mainly influenced by the amount of clay and clay mineral, water saturation and thickness of cap beds. The amount of clay and clay mineral play a decisive role in the capillary sealing capacity and the high water saturation enhances  the capacity of capillary sealing. In addition, the thickness of cap beds plays a compensation role on the capacity of capillary sealing.