A flame retardant toughening agent, polyethylene glycol borate(PEGB), was synthesized from the esterification reaction between polymer toughening agent of polyethylene glycol(PEG) and flame retardant boric acid. Effects of reaction temperature, entrainer amount and PEG/BA molar ratio were studied, and the optimum reaction conditions were determined. The structure of PEGB was characterized by Fourier transform infrared spectroscopy(FTIR). Phenolic foams(PF) modified by PEGB with different relative molecular masses were prepared. The results of foam property tests show that the mechanical strengths of PF/PEGB are higher than those of pure PF, and the flame retardancy of PF/PEGB is also improved compared with PF/PEG. Particularly, PEGB400 performs best for increasing flexural strength and compressive strength to 0189，0108MPa respectively, keeping the limited oxygen index(LOI) to 380%, and achieving UL 94 rating of V0. Such performance is qualified for thermal insulation building materials.