Poly($gamma$-benzyl L-glutamate)-graft-poly(ethylene glycol) (PBLG-graft-PEG) copolymer was synthesized by the ester exchange reaction of PBLG homopolymer with PEG chain. Chemical structure of PBLG-graft-PEG was confirmed by nuclear magnetic resonance (NMR) spectroscopy, morphology of copolymer membrane was investigated using scanning electron microscopy (SEM), and thermal transition behavior was studied using differential scanning calorimeter (DSC). It was revealed that PBLG-graft-PEG copolymer holds different structure and performance compared with PBLG homopolymer. The introduction of PEG chains not only decreases the melting point of PBLG segments in PBLG-graft-PEG, but also changes the morphology of PBLG segments. Influence of grafting ratio on chemical and mechanical properties of PBLG-graft-PEG copolymer membrane was studied.