The invention relates to the field of raw material gas purification in the ammonia synthesis production process and discloses a purification device and a purification method for ammonia synthesis raw material gas. The device comprises a pre-cooling heat exchanger, a copious-cooling heat exchanger, a demethanizing tower and a methane recovering tower, wherein the ammonia synthesis raw material gasis sequentially connected with the pre-cooling heat exchanger, the copious-cooling heat exchanger, the demethanizing tower and the methane recovering tower through pipelines, synthesis gas with the hydrogen-to-nitrogen mixture ratio being 3:1 is obtained at the tower top of the demethanizing tower, and products are obtained at the tower bottom of the methane recovering tower. The purification device and the purification method have the advantages that the problem of too high exhausted gas emission quantity of ammonia synthesis plants adopting the methanation process can be solved, and simultaneously, products can be obtained.