The invention discloses a method for recycling a waste mercury catalyst. The method comprises the steps of adding the waste mercury catalyst into water, controlling the temperature at 70-95 DEG C, carrying out hot dissolution pretreatment while stirring, regulating the pH value to 1-3, and filtering after carrying out ultrasonic treatment for 10-60min or filtering in an ultrasonic field to separate mercuric chloride and phosphorus and sulfur impurities from activated carbon in time, wherein the mercuric chloride and the phosphorus and sulfur impurities are separated from the surface of the activated carbon; after filtering, washing the obtained activated carbon and sieving to prepare a mercury catalyst; controlling the temperature of mercury-contained filtrate at 50-90 DEG C, adding a neutralizing agent to regulate the pH value to 6-8, and reacting for 30-90min, wherein mercury ions in the solution are completely converted into mercuric oxide sediments, and toxic substances, namely phosphorus and sulfur in the mercury catalyst still remain in the solution; filtering to obtain a mercuric oxide filter cake, and dissolving with hydrochloric acid to obtain a mercuric chloride solution for preparing the mercury catalyst. According to the invention, the waste mercury catalyst is recycled through acidification-ultrasonic synergistic desorption, coupling and filtration, therefore, the method is simple in process, high in efficiency and low in energy consumption.