The invention relates to an active group containing polyacrylate flexibilizer, a preparation method thereof and epoxy resin adhesive modified by the flexibilizer. The invention solves the problems that the existing polyacrylate modified epoxy resin adhesive has poor heat resistance, shear strength and peel strength are low, and the existing adhesive can not be used as structural adhesive. The flexibilizer is obtained by reaction of monomer, dispersion medium, initiator, chain transfer agent and chain terminator. Half of mixed solution of monomer and initiator and dispersion medium are added into a reactor, and then the rest of mixed solution and the chain transfer agent are added into the reactor, and finally the chain terminator is added, thus obtaining the flexibilizer by reaction. The adhesive contains component A and curing agent as well as component C DMP-30; wherein the component A is composed of flexibilizer and epoxy resin. The flexibilizer modified adhesive of the invention has good heat resistance, shear strength at 25 DEG C can reach 44.7MPa, and peel strength at 25 DEG C can reach 8.24kN / m. The adhesive of the invention can be used as structural adhesive.