ObjectiveTo investigate the effect and mechanism of polysaccharide in water extract of Cyclocarya paliurus （CPWP） in inhibiting benign prostatic hyperplasia （BPH）. MethodsCPWP was obtained by heating reflux， aqueous extraction， alcohol precipitation， and freeze drying. The chemical composition and structural properties of CPWP were analyzed by high performance liquid chromatography with 1-pheny-3-methyl-5-pyrazolone pre-column derivatization and infrared spectroscopy. Male SD rats were randomly assigned into control， model， finasteride （ig 5 mg·kg^-1）， and low-， medium-， and high-dose （ig 50， 75， 100 mg·kg^-1） CPWP groups， with 8 rats in each group. The BPH model was established by subcutaneously injecting propionate testosterone in castrated rats. The rats in the drug intervention groups were administrated with corresponding drugs， and those in the control group were administrated with an equal volume of normal saline each day. After 30 consecutive days， the rats were sacrificed， and the prostate tissue was separated and weighed. The effects of drug interventions on the body weight， prostate wet weight， and prostate index of rats were examined. The prostate tissue was stained with hematoxylin-eosin （HE） for observation of pathological changes. Enzyme-linked immunosorbent assay was employed to measure the level of dihydrotestosterone （DHT）， and immunohistochemical staining was used to detect the expression of steroid 5 alpha-reductase 2 （SRD5A2） and Ki67 in the prostate tissue. ResultsCPWP was identified as a saccharide， with characteristic absorption peaks of saccharides. CPWP showed the total sugar content of 44.15% and molecular weight within the range of 5.5-78.8 kDa， being composed of mannose， rhamnose， galacturonic acid， glucose， galactose， xylose， and arabinose. Compared with the control group， the model group had significantly increased prostate wet weight and prostate index （P<0.01）， thick and tall prostate epithelial cells， increased internal wrinkles， papillary expansion into the cavity， an elevation in DHT level in the serum， and up-regulated expression of SRD5A2 and Ki67 in the prostate tissue （P<0.05， P<0.01）. Compared with the model group， both the finasteride and CPWP groups showed decreases in prostate wet weight and prostate index （P<0.05， P<0.01）， thinned prostate epithelial cells， with only a small portion of internal wrinkles and papillary expansion into the cavity， shortened papillary protrusions， lowered DHT level in the serum， and down-regulated expression of SRD5A2 and Ki67 in the prostate tissue （P<0.01）. Moreover， CPWP exerted effects in a dose-dependent manner. ConclusionCPWP inhibits BPH by regulating the expression of SRD5A2.