An active component of tree peony bark is just benzoylpaeoniflorin. Its preparing process includes such steps as breaking red sage root, adding ethyl acetate and alcohol, thermal extracting, concentrating to obtain extract, mixing it with silicon gel, separating with positive-phase silicon gel column, washing with petroether and ethyl acetate, eluting with chloroform and methanol, concentrating the eluting liquid, drying, and separating by liquid-phase chromatography. Its application is also disclosed.