Hydroxy-terminated hyperbranched polyester (HBPE) was synthesized via a one-step method by using trimethylolpropane (TMP) as the core and 2,2-dimethylolpropionic acid (DMPA) as the monomer. Subsequently, HBPE was added to polycarbonate (PC) as a modifier. The PC/HBPE blends were characterized by Fourier-transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TG), universal tensile testing machine, rotational rheometer, and melt flow indexer (MFR). The results indicated that with the addition of HBPE, the glass transition temperature of the PC/HBPE blends was decreased from 147.5 ℃ to 130.7 ℃, demonstrating that the flexibility of PC chains was enhanced by the addition of HBPE. When the content of HBPE is 2 phr, a 7.1% increase in fracture strength (to 64.6 MPa) of the PC/HBPE blend compared to pure PC is observed; when the HBPE content is 5 phr, increases of 29.4%, 136.8%, and 59.3% in elongation at break, notched impact strength, and tensile toughness of the PC/HBPE blend compared to pure PC are seen respectively. Melt flow index testing revealed that when the HBPE content was 5 phr, the melt flow index was increased by 288.4% compared to pure PC, suggesting that the melt viscosity of PC was reduced by the addition of HBPE. Therefore, HBPE is an excellent toughening, strengthening, and plasticizing modifier, with broad application prospects.