Abstract: Using polyethylene glycols(PEG) with different relative molecular masses as additives and N,N dimethylacetamide (DMAc) as solvent, the phase behavior of polyetherimide (PEI) and polyethylene glycol was discussed. The mixability parameters of water, methanol, ethanol and ethylene glycol in w =0.02 solids of PEI/DMAc have been measured. The mixability parameters of different nonsolvents have been found to be in the following order: ethanol>methanol>ethylene glycol>water. Using water as coagulant, polyetherimide (PEI) ultrafitration membranes were prepared. The water permeation fluxes and the separation property of the membranes for BSA were investigated. According to Scanning Electron Microscope (SEM), effects of different relative molecular masses of polyethylene glycol (PEG) on membrane structures were analysed.