The invention discloses a silver-loaded lignite semi-coke used for absorbing elemental mercury from coal-fired flue gas. The silver-loaded lignite semi-coke has a specific surface area being above 280 square meters per gram and a silver-loaded content being 0.01%-0.3%. Silver particles are loaded in surface and ducts of the silver-loaded lignite semi-coke, wherein the diameters of the silver particles are less than 5 nanometers. A preparation method for the silver-loaded lignite semi-coke comprises the following steps: taking lignite as a raw material; soaking the lignite in a H2O2 solution and then placing the lignite in an AgNO3 solution; reacting under an ultrasonic shaking condition; and after finishing the reaction, performing thermal decomposition and steam activation, thereby acquiring an end product. According to the method, by loading a certain amount of nanometer silver particles on a cheap lignite semi-coke matrix, the mercury pollutant can be efficiently adsorbed from the coal-fired flue gas. Besides, an absorbing agent has the advantages of large saturation adsorption volume, low cost, regeneration and reusability, and the like.