ABA-type block copolymers were synthesized by atom transfer radical polymerization (ATRP). The molecular structure and composition of block copolymers were determined by nuclear magnetic resonance spectroscopy (NMR), gel permeation chromatography (GPC) and Fourier transform infrared spectroscopy (FT-IR). The epoxy resin/ diaminodiphenyl sulfone (EP/DDS) system was modified with block copolymer. The thermal stability, rheological properties, mechanical properties, microstructure, compatibility and toughening mechanism of epoxy resin were investigated. The results show that the amount of acrylate triblock copolymer is 10 phr, the comprehensive performance of the blends is the best. The curing reaction process is fixed as 150 ℃×1 h+180 ℃×3 h. The modified system also maintains a wide temperature processing window, which can be used as resin matrix of composites. In addition, SEM fracture surfaces emerge a large number of jags resulting in an effectively hindrance to the expansion of cracks. Thereby, the toughness of epoxy resin is enhanced.