The invention relates to a plaster for treating benign prostatic hyperplasia and belongs to a pure Chinese medicinal preparation. The plaster comprises the following components in part by weight: 1 to 5 parts of common monkshood mother root, 1 to 5 parts of kusnezoff monkshood root, 1 to 5 parts of rhubarb, 1 to 5 parts of cassia bark, 1 to 5 parts of nutgrass galingale rhizome, 1 to 5 parts of pangolin scales, 1 to 5 parts of root of red-rooted salvia, 1 to 5 parts of largehead atractylodes rhizome, 1 to 5 parts of glabrous greenbrier rhizome, 1 to 5 parts of centipede, 1 to 5 parts of Szechuan lovage rhizome, 1 to 5 parts of white paeony root, 1 to 5 parts of Chinese angelica, 1 to 5 parts of raw rehmannia root, 1 to 5 parts of atraotydin, 1 to 5 parts of Himalayan teasel root, 1 to 5 parts of glossy privet fruit, 1 to 5 parts of silktree albizzia bark, 1 to 5 parts of liquoric root, 400 to 600 parts of pure soybean oil and an appropriate amount of starch. A method for preparing the plaster comprises the following steps of: soaking the medicaments into the pure soybean oil; putting the medicaments into a sandy pot after soaking for enough days; decocting the medicaments over slow fire until the medicaments are dried and floated; filtering gruffs off with disinfected white cloth to prepare medicinal oil; making the ratio of the medicinal oil to the starch be 2:1; decocting the mixture over slow fire; continuously stirring the mixture until the mixture becomes sticky; dropping the mixture into water to become beads; collecting the beads into a porcelain container; and soaking the porcelain container into cold water for later use. The plaster has obvious effects of resisting inflammation, promoting urination and relieving pain because of compatibility of the Chinese medicaments, can be directly pasted on an affected part, and has 97.1 percent of significant efficiency.