High filler content is the prerequisite for imparting specific properties to polymeric composite. However, serious mechanical properties degradation happened in this kind of material due to the inevitable reduction of material cohesion. In this work, highly filled poly(butylene terephthalate)/ alumina (PBT/Al2O3) composites were modified by directly melt mixing with a reactive compatibilizer ethylene–methyl acrylate–glycidyl methacrylate terpolymer (E-MA-GMA). The modified samples presented greatly enhanced elongation at break and impact response, simultaneously preserved their tensile strength and high thermal conductivity. The reactive compatibilization between PBT and Al2O3 spheres was confirmed by SEM, FTIR, and rheology characterizations. Three rheology criteria plots demonstrated that the filler percolating network and apparent yield behavior occurred in the ternary PBT/Al2O3/E-MA-GMA composites melt. It is believed that enhanced particle-matrix interfacial interaction and stress transfer caused by E-MA-GMA endowed this highly filled composite improved toughness. This study suggests the potential application of E-MA-GMA elastomer to resolve the mechanical properties degradation in highly filled polymeric composites.