The invention relates to a system and method for collaborative removal of mercury and sulfur dioxide from flue gas. The system can utilize the existing traditional flue gas desulfurization tower or washing tower, in which a slurry pool, an inlet flue, a slurry spray layer, a lower washing layer, a composite adsorption catalyst layer, an upper washing layer, a demister and an outlet flue are arranged from the bottom up in order. According to the invention, a composite adsorption catalyst layer and a washing layer are added above the slurry spray layer, while the composite adsorption catalyst adsorbs mercury in flue gas, sulfur dioxide in the flue gas is catalyzed for oxidization into sulfuric acid liquid drops, and then the generated sulfuric acid liquid drops are discharged from the composite adsorption catalyst layer by means of spray washing. Compared with other traditional flue gas mercury removal methods like adsorbent jet technology or activated coke/activated carbon dry process integration technology, the system has the advantages of simple structure, easy installation, small floor area and low operation cost, and also has the function of further improving the removal rate ofsulfur dioxide.