In order to study the effect of the addition of ［N2，2，2，2］BF4 on the phase formation behavior of PEG-NaH2PO4· 2H2O aqueous two phase system，firstly，［N2，2，2，2］BF4 was synthesized by orthogonal optimization method and character ized by 1H NMR and its purity was verified.Secondly，under the condition of 25 ℃，the phase diagrams of PEG-NaH2PO4· 2H2O and PEG-salt-4%［N2，2，2，2］BF4 aqueous two phase systems were plotted by cloud point method respectively.Finally， the effect of［N2，2，2，2］BF4 with 4%（mass fraction） addition on the phase formation behavior of PEG-NaH2PO4·2H2O aque ous two phase system was studied.The results showed that，through single factor experiment and L9（34） orthogonal optimiza tion experiment，the optimum conditions of the synthetic reaction were obtained as follows：the amount of substance ratio of［N2，2，2，2］Br to NaBF4 was 1.0∶1.1，the amount of ACN was 100 mL，the reaction time was 8 h，the temperature was 80 ℃ and the yield of ［N2，2，2，2］BF4 was up to 90.7%.The Merchuk equation was used to correlate the phase diagram data of the two systems，the fitting degree of them were more than 0.997.With the increase of the relative molecular mass of PEG，the area of the aqueous two phase region increased gradually by the addition of 4%［N2，2，2，2］BF4 instead of PEG partly，which enlarged the selection range of the phase forming agent and is beneficial to the selection of the aqueous two phase systems and laid a foundation for the subsequent separation of biomolecules in aqueous two phase system.