Poly(butylene succinate) (PBS)/soy protein isolate (SPI) bio-composites were reactive compatibilized by adding dibenzoyl peroxide (BPO) and hexanediol diacrylate (HDDA) via melt blending in an internal mixer. The structure and properties of composites were studied by scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FTIR), Soxhlet extraction experiments, dynamic mechanical analyses (DMA), rheological tests, contact angle measurements, thermogravimetric analyses (TGA), differential scanning calorimetry (DSC), wide-angle X-ray diffraction (WAXD), mechanical tests and water absorption tests. The results show that a branched graft copolymer (SPI-g-HDDA-g-PBS) was produced with the aid of BPO and HDDA via melt blending, which served as the compatibilizer and improved the compatibility and enhanced the adhesion between PBS matrix and SPI phase by forming a network structure in the composites. The crystal form of PBS was not changed in the composites. The melt viscocity and elasticity, hydrophobicity, thermal stability, crystallinity, tensile strength and water resistance of PBS/SPI-HDDA/BPO composites were improved compared with PBS/SPI composites.