The invention relates to the technical field of gas separation, in particular to a separation method for separating methane with the low concentration being smaller than 30% in nitrogen and an applieddevice, and specifically discloses a method and device for enriching methane by separating nitrogen. The method comprises the steps that methane-nitrogen mixed gas passes through an adsorption bed A,adsorbent containing TUTLY-1 selectively adsorbs methane gas, after adsorption bed A methane adsorption is saturated, the adsorption bed A is subjected to decompression or vacuum desorption, desorbedgas subjected to first-level concentration serves as feed gas to enter an adsorption bed B, and adsorbent containing TUTLY-2 selectively adsorbs nitrogen. Accordingly, the methane loss ratio in the mixed gas is low, the equipment investment is low, starting and stopping are flexible, and operation is easy and convenient; the gas concentration of obtained methane can reach 95% or above, and meanwhile high-purity nitrogen (larger than 95%) can be obtained. High methane gas with the methane concentration being 95% or above can be directly used as LNG fuel or natural gas for vehicles.