The invention discloses a plaster for treating chloasma and a preparation method thereof, and belongs to the technical field of medicines for treating chloasma. The plaster comprises the following Chinese herbal medicines in part by weight: 10 parts of chestnut kernel, 5 parts of dried loofah, 25 parts of rehmannia root, 15 parts of hiraute shiny bugleweed herb, 15 parts of motherwort herb, 12 parts of cicada shell, 10 parts of stiff silkworm, 10 parts of Chinese angelica, 10 parts of red paeony root, 10 parts of peach kernel, 10 parts of safflower, 10 parts of Chinese thorowax root, 10 parts of tree peony bark, 10 parts of tuckahoe, 10 parts of oriental waterplantain rhizome, 6 parts of szechuan lovage rhizome, 12 parts of cornus officinalis, 12 parts of cinnamomvine, 15 parts of sinkiang arnebia root and 4.5 to 15 parts of aloe. The plaster has the obvious effects of whitening, removing chloasma, resisting wrinkles and the like, has good curative effects on chloasma caused by heredity, insolation, pregnancy, administration of medicines, chronic diseases, improper using of cosmetics and dietetic factors; and the effective rate reaches more than 85 percent.