Three-dimensional (3D) composite porous scaffolds containing hydroxyapatite (HA) and polymer matrix showed wide applications in bone repair because of their improved biocompatibility, bioactivity, and mechanical properties in our previous studies. In this work, mesoporous hydroxyapatite (MHA) surface-modified by poly(γ-benzyl-l-glutamate) (PBLG) with different amounts (from 11 to 50 wt %) was synthesized by the in situ ring opening polymerization of γ-benzyl-l-glutamate N-carboxy anhydride (BLG-NCA) and then PBLG-g-MHA/PLGA composite films were prepared to illustrate the biological performance of the composites. Furthermore, porous scaffolds of PBLG-g-MHA/PLGA were fabricated through modified solvent casting/particulate leaching (SC/PL) method to demonstrate the ability of in vivo bone defect repair. In vitro cytological assay indicated that enhanced cell expansion on PBLG-g-MHA/PLGA with 11 wt % PBLG amounts and improved osteogenic differentiation on the composites with 33 and 50 wt % PBLG amounts were achieved. And the porous scaffolds exhibited high porosity and interconnected pores. Results of the in vivo rabbit radius defect repair indicated that rapid mineralization and new bone formation could be observed on the composites with 22 and 33 wt % PBLG. This study revealed that PBLG-g-MHA/PLGA composites might have potential applications in clinical bone repair.