Disclosed is a ruthenium-based catalyst for ammonia synthesis, preparation method and use thereof. The ruthenium-based catalyst comprises Ru—Ba-A core-shell structure which comprises a ruthenium nanoparticle as a core covered with a first shell and a second shell sequentially, wherein the first shell consists of a barium nanoparticle, and the second shell consists of a metal oxide. The Ru—Ba-A core-shell structure can effectively preventing agglomerations of ruthenium nanoparticles during the use of the catalyst and avoiding direct contact between the ruthenium nanoparticles and the metal oxides. In addition, barium nanoparticles have a promoting effect as an electronic promoter, which can effectively improve the stability and catalytic activity of ruthenium-based catalyst for ammonia synthesis, especially in the system for synthesizing ammonia from a coal gas.