Polysaccharide's 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-MgSO-HO, which succeeded in one-step CCC separation of a polysaccharide (43mg) from the water extract (1.67g) of Radix Astragali. The solvent composition was set as 12:16:72 (w/w/w) of which the lower phase was used as the mobile phase at a flow rate of 0.8mL/min in a 1000mL column. The purified polysaccharide bears an average molecular weight of 1095kDa 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-linked Glcp, 11.8% of T-linked Araf, 10.5% of T-linked Glcp, 9.1% of 1,4,6-linked Galp and 5.1% of 1,3,6-linked 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. © 2012 Elsevier B.V.