The invention provides a preparation method for flue gas mercury-removing active carbon. The preparation method comprises the following steps: step 1, removing a surface chemical group of a raw material: washing the particle active raw material with 10.wt%-30.wt% NaOH solution and drying the raw materials at 100-120 DEG C; step 2, immersing halide salt: mixing, immersing and agitating the raw material active carbon with an oxygen-containing group on the surface and a halide salt solution according to the mass ratio ranging from 1:1 to 1:5 for 4-12 hours; filtering and drying to obtain active carbon carrying the halide salt, wherein the concentration of the halide salt solution is 10wt.%-50wt.%; step 3, loading halogen: putting the active carbon carrying the halide salt into a U-shaped pipe to form a filling layer; inputting air with the halogen from the top of the filling layer at the speed of 100-500mL/min and keeping the heat for 1-10 hours at room temperature to obtain saturated halogen-loaded active carbon; and step 4, heating the saturated halogen-loaded active carbon at the temperature in a range of 80-120 DEG C to remove the volatile halogen to prepare the flue gas mercury-removing active carbon.