OBJECTIVE： To conduct sulfated modification of polysaccharide from Dictamnus dasycarpus （DDP-Ⅲ）， and to compare structure characteristics and anti-psoriasis activity of DDP-Ⅲ before and after sulfated modification. METHODS： DDP-Ⅲ was separated and purified with DEAE-52 anion exchange cellulose column and Sephadex G-100 column. After derived with 1-phenyl-3-methyl-5-pyrazolone， HPLC was used to determine the composition of its monosaccharide. SDDP-Ⅲ was synthesized using esterification reagent （anhydrous pyridine+chlorosulfonic acid） to modify DDP-Ⅲ. The degree of sulfate substitution was determined by barium chloride-gelatin turbidimetric method. The structures were compared by IR， Raman spectrum and SEM before and after modification. The male ICR mice were randomly divided into normal group， model group， positive group （tripterygium glycosides， 20 mg/kg） and DDP-Ⅲ/SDDP-Ⅲ low-dose， medium-dose and high-dose groups （56， 112， 224 mg/kg）. Except that normal group was given vaseline for external use， and other groups were given Imiquimod cream for external use to induce psoriasis model. At the same time， administration groups were given relevant medicine intragastrically 0.4 mL， and both normal group and model group were given constant volume of water intragastrically， once a day， for consecutive 14 d. Two hours after last medication， the serum contents of IL-17 and IL-23 were determined by ELISA. The skin scales near the tail were observed by HE staining， and the number of scales with granular layer was recorded. RESULTS： DDP-Ⅲ was composed of mannose， rhamnose， glucuronic acid， galacturonic acid and glucose. The degree of sulfate substitution was 0.65 for SDDP-Ⅲ. IR and Raman spectrum showed that the characteristic absorption peaks of sulfate radical group appeared near 1 255 cm－1 and 823 cm－1， 1 240 cm－1 and 815 cm－1 for SDDP-Ⅲ， except for same characteristic absorption peak as DDP-Ⅲ. SEM analysis showed that DDP-Ⅲ was flaky， smooth and tightly arranged； SDDP-Ⅲ was massive or granular with porous structure and loose arrangement. Animal experiment showed that compared with normal group， the epidermis of skin lesion was significantly thickened and the granular layer was significantly reduced； serum contents of IL-17 and IL-23 were increased significantly， while the number of scales with granular layer was decreased significantly （P＜0.05 or P＜0.01）. Compared with model group， above symptoms of administration groups were improved to different extent， and serum contents of IL-17 and IL-23 in positive group， DDP-Ⅲ high-dose groups， SDDP-Ⅲ medium-dose and high-dose groups were decreased significantly； the number of scales with granular layer was increased significantly， and above indexes of SDDP-Ⅲ medium-dose and high-dose groups were significantly better than corresponding DDP-Ⅲ group （P＜0.05 or P＜0.01）. CONCLUSIONS： DDP-Ⅲ contains five monosaccharide components such as mannose， etc. Both DDP-Ⅲ and SDDP-Ⅲ possess anti- psoriasis effects， and SDDP-Ⅲ exhibits stronger anti-psoriasis effect than DDP-Ⅲ. Its mechanism may be associated with inhibiting IL-23/IL-17 signaling pathway.