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Glycidyl methacrylate (GMA) and styrene (St) were used as the graft monomer and co-graft monomer, respectively, and grafted onto poly(ethylene-octene) (POE) and linear low-density polyethylene (LLDPE) to prepare a series of GMA-functionalized (POE/LLDPE)-g-(GMA-co-St) graft copolymers. The copolymers were then blended with poly(butylene terephthalate) (PBT) to produce PBT/(POE/LLDPE)-g-(GMA-co-St) blends. The effects of grafting factors and copolymer content on the mechanical properties, thermal properties, rheology, morphology, and deformation mechanism of the blends were studied in detail. The results showed that GMA and St were successfully grafted onto the molecular chains of POE and LLDPE. In the grafting reaction, the best grafting degree and comprehensive performance were achieved when the amounts of dicumyl peroxide and GMA were 0.3 and 5 wt %, respectively, the mass ratio of GMA to St was 1:2, and the screw rotation speed was 60 rpm. By introducing graft copolymers into the PBT matrix, the toughness and thermal stability of the blends were improved. At the same time, the crystallinity and fluidity decreased, and the blends possessed the best mechanical properties when the copolymer content was 20 wt %, with the elongation at break and notched impact strength increasing to 409.98% and 945.90 J/m, respectively. Scanning electron microscopy analysis showed that the copolymers were uniformly dispersed in the PBT matrix. The principal toughening mechanisms were the shear-yielding properties of the PBT matrix and cavitation of (POE/LLDPE)-g-(GMA-co-St) phases.