Norcantharidin is an antitumor drug first synthesized in China, which has effects such as increasing white blood cells, protecting liver cells, and regulating immune function. Its anti-cancer mechanism is gradually being elucidated. However, due to its still certain organ toxicity, it limits the clinical dosage used, affecting the anti-cancer effect; currently, the norcantharidin marketed domestically does not have biodegradability and also lacks good sustained-release properties, still having the disadvantages of high toxicity and frequent medication. Overseas research on norcantharidin is mainly in the pharmacological field, with no research on dosage forms remaining a blank. This invention proposes a norcantharidin sustained-release microsphere and its preparation method. The drug encapsulated within the microsphere is norcantharidin, and the drug carrier is a biodegradable polymer with a weight average molecular weight of 5000-50000, capable of stable release for more than 60 hours. In terms of preparation, Use a polymer solution of dichloroethane or ethyl acetate as the oil phase, and add a surfactant-containing aqueous solution of norcantharidin as the water phase. Mix the two phases evenly to form an initial emulsion; then add the initial emulsion to an aqueous solution containing polyvinyl alcohol, stir or evaporate to obtain a secondary emulsion; dialyze the secondary emulsion to remove free drug; finally freeze dry it into powder and store sealed.