Abstract: A hydrate of dimethylaminomicheliolide fumarate has a crystal form D, is high in crystallinity, smooth in particle surface, free of coalescence, high in bulk density and good in flowability, and has good stability and good reproducibility. Its preparation method includes: under a stirring action, adding dimethylamine micheliolide and fumaric acid into a mixed solvent system having a constant temperature of 30° C.-70° C. for reaction crystallization; and filtering after the reaction is finished, and drying filtered solid at normal pressure to obtain the crystal form D of the dimethylaminomicheliolide fumarate.