The invention provides a preparation method of a copper and cerium composite silver halide/silver photocatalyst. The preparation method of the copper and cerium composite silver halide/silver photocatalyst comprises the following steps of mixing a ready Ce(NO3)3 solution with a Cu(NO3)2 solution, and then adding citric acid into an obtained first mixture, and heating insolating, grinding and sieving an obtained second mixture in sequence to obtain solid powder CuyCe1-yOz; adding the solid powder CuyCe1-yOz into an AgNO3 solution to obtain a solution A, and then adding a Na2CO3 solution into the solution A; afterwards, adding 0.1 mol of potassium chloride (KCl), potassium bromide (KBr) or potassium iodide (KI) into an obtained third mixture to obtain a solution B; culturing the solution B through a heating device, and then adding ethanediol into the cultured solution B to obtain a solution C; washing, drying, grinding and sieving the solution C to obtain the copper and cerium composite silver halide/silver photocatalyst. A flue gas demercuration method adopting the copper and cerium composite silver halide/silver photocatalyst comprises the following steps of making reaction liquid containing a copper and cerium codoped silver halide/silver photocatalyst; irradiating the reaction liquid and a mercury-containing flue gas by visible light to carry out demercuration. The preparation method of the copper and cerium composite silver halide/silver photocatalyst and the flue gas demercuration method adopting the copper and cerium composite silver halide/silver photocatalyst have the characteristics of good demercuration effect, low cost and the like, and can be widely applied to the field of atmospheric controlling.