Human serum albumin (HSA) was used widely in medical and biological fields. The HSA was extracted by an aqueous two-phase system composed of PEG and (NH4)2SO4，and the distribution of HAS was investigated. The results showed that both partition coefficient and extraction yield of HAS rose up with the increase of PEG molecular weight,and the concentration of PEG and (NH4)2SO4, and they got to the optimal point at 19.53 % of PEG, 14.07 % of (NH4)2SO4, but they went down at both sides. The partition coefficient and extraction yield of HSA were also increased with the increase of salt concentration, and reached maximum at 10 % NaCl.