The invention relates to an extraction and preparation method of bioactive components of a traditional Chinese medicine, in particular to 5-alpha reductase inhibitor-smartweed diffractive ring lignan glucoside B and a preparation method thereof. In the invention, a triangle-leaf smartweed is separated to obtain smartweed diffractive ring lignan glucoside B on the basis of previous research, the component has a structure different from a lignans component and is a novel diffractive ring lignans component, and a molecular structural formula of the novel diffractive ring lignans component is as above; and the preparation method of the 5-alpha reductase inhibitor-smartweed diffractive ring lignan glucoside B takes the triangle-leaf smartweed as a raw material. The 5-alpha reductase inhibitor-smartweed diffractive ring lignan glucoside B is discovered to have higher activity on the research of inhibiting 5-alpha reductase action and is a potential prostatic hyperplasia resistant medical component.