The invention relates to a plaster and a preparation method thereof, in particular relating to a liquid plaster for treating osseous rheumatism and a preparation method thereof. The liquid plaster is prepared from the following medicinal materials in parts by weight: radix saposhnikoviae, Chinese pangolin, red-rooted salvia roots, angelica sinensis, rhizoma dioscoreae nipponicae, ligusticum wallichii, teasel root, pseudo-ginseng, dragon's blood, cructus chaenomelis, eucommia ulmoides, radix achyranthis bidentatae, radix clematidis, caulis sinomenii, notopterygium root, radix angelicae pubescentis, radix aucklandiae, rhizoma gastrodiae, safflowers, peach kernel, cassia twig, radix paeoniae alba, Chinese ephedra, asarum, mastic, myrrh, radix aconiti agrestis, radix aconiti kusnezoffii, radix zanthoxyli and caulis piperis futokadsurae, and the preparation process is simple and practicable. The liquid plaster is short for treating process, easy to absorb, and low in cost, can be used for continuous treating, and is wide in application prospect.