Ethylene–methacrylate–glycidyl (EMG) copolymer is employed to strengthen polyethylene terephthalate (PET)/glass fiber (GF) blends. This paper starts from investigating the effects of various EMG contents on mechanical properties, thermal properties and fractured surface morphology of PET/GF blends. All of the above-mentioned properties own extreme limits of EMG concentration. The crystallization ability of the blends increases with an increment in EMG content, whereas the crystallinity keeps stable at a relatively high level of 0–20 wt.% EMG loading. The tension, bending and impact properties of PET blends are enhanced with the addition of a self-made three-dimensional hierarchical porous carbon sponge (3DC) based on an optimal additive amount. Results indicate that EMG possesses the capabilities of increasing the toughness of PET/GF blends remarkably and transforming the blends from brittle fracture to tough fracture. According to the results, the blends exhibit the best overall properties as the content of EMG reaches 10–15 wt.%.