The invention relates to a catalyst for synthesizing ammonia through activated-carbon-loading ruthenium, and a preparation method thereof. The method comprises: adopting activated carbon subjected to high-temperature treatment and oxidizing-reaming treatment as a carrier; using Ba (NO3)2, Mg (NO3)2, and Ln (NO3)3 aqueous solution to modify the activated carbon carrier by an impregnation method according to the molar ratio of Ba to Mg to Ln being 1:1-3:0.01-1, wherein Ln is a mixture of one or more than two of La, Pr, Nd and Sm; plating the carrier with RuCl3 by the impregnation method; using hydrogen or hydrogen-nitrogen mixed gas to reduce the RuCl3 into metallic ruthenium at a temperature of between 110 and 470 DEG C; and plating the obtained product with KNO3, K2CO3 or KOH additives bythe impregnation method. Based on the weight of the activated carbon carrier, the catalyst consists of 2 to 7 weight percent of Ru, 3 to 8 weight percent of Ba+Mg+Ln, and 6 to 18 weight percent of K.The catalyst is applicable to synthesizing ammonia from hydrogen and nitrogen under the reaction pressure of between 5 and 20 MPa at a reaction temperature of between 230 and 475 DEG C.