The invention discloses an external device and a method for enriching denitrification anaerobic methane oxidation bacteria by using a methane bubbleless aeration biomembrane. The device comprises a reactor main body, a methane bubbleless aeration membrane assembly, a temperature control system, a stirring system and a liquid sealing device; the reactor main body is provided with a reactor cover plate, a reaction container, a biological filler, an effluent biomass interception sphere, a circulation liquid biomass interception sphere, and a biological filler support, wherein the reactor main body is divided into a biological filler area and a complete mixing area by a support plate of the biological filler support; the biological filler area comprises the biological filler, the effluent biomass interception sphere, and the circulation liquid biomass interception sphere; the complete mixing area comprises support columns of the biological filler support, a stirring magneton and a water outlet of a circulating liquid water inlet pipe; and a hollow fiber membrane is arranged in the methane bubbleless aeration membrane assembly. The dissolution velocity of methane in an aqueous phase isimproved by a methane bubbleless aeration technology, and the running efficiency of a reactor is improved by a biomembrane technology.