The invention relates to a preparation method of a plant compound monomer, and belongs to the technical field of separation and purification of traditional Chinese medicine active ingredients. The preparation method comprises the steps of: carrying out alcohol extraction on Alisma orientalis (Sam.) Juzep tuber serving as a raw material, extracting with ethyl acetate, enriching by silica gel column chromatography, adding ethyl acetate, and cooling and crystallizing to obtain the alisol B 23-acetate monomer. In the invention, by optimal process and parameter conditions, the content of alisol B 23-acetate in the product achieves more than 98%; and the entire process is simple and convenient to operate, stable in process, low in cost, high in separation efficiency and high in product purity, and is suitable for mass production. Therefore, high-purity separation and preparation of massive alisol B 23-acetate monomer are realized.