Polysaccharides purification remains challenge to separation technology.Conventional methods involve time-consuming and complicated operations and always cause significant variation in the isolates chemistry.This paper reports an aqueous diphase solvent system,namely PEG1000-MgSO4-H2O,which succeeded in one-step CCC separation of a polysaccharide (43 mg) from the water extract (1.67 g) of Radix Astragali.The solvent composition was set as 12∶16∶72 (w/w) of which the lower phase was used as the mobile phase at a flow rate of 0.8 mL/min in a 1000 mL column.The purified polysaccharide bears an average molecular weight of 1095 kDa and consists of galacturonic acid (76.5%),galactose (7.7%),arabinose (4.2%) and glucose (5.0%).Methylation analysis result showed it was composed of 58.4% of 1,4-1inked Glcp,11.8% of T-linked Araf,10.5% of T-linked Glcp,9.1% of 1,4,6-1inked Galp and 5.1% of 1,3,6-1inked Galp,etc.This success shows a short way between the crude water extract and purified polysaccharides,which minimizes the chemical variation caused by purification methods.