A two-step procedure was used to synthesize the cellulose acetate butyrate and poly(ethylene glycol) graft copolymer (CAB-g-PEG). By choosing the appropriate composition, the crosslinked graft copolymer or not could be obtained. Then, the CAB-g-PEG copolymer was blended with poly(3-hydroxybutyrate) (PHB), to further improve the mechanical properties of PHB. The results indicated that PHB and CAB-g-PEG that were not crosslinked were miscible over the entire composition range. As the CAB-g-PEG copolymer increased in the PHB/CAB-g-PEG blends, the melting temperature of the blends decreased, the crystallization of PHB became more difficult, and the crystallinity of the blend and PHB phase all decreased. The tensile properties and impact strength of the PHB/CAB-g-PEG blends were superior to the PHB/CAB blends.