The triblock copolyether of hydroxyl-terminated polyallyl glycidyl ether (PAGE)/polytetrahydrofunan /PAGE (PAGE-PTMEG-PAGE)was synthesized via. the cationic ring-opening polymerization, using polytetrahydrofunan ether glycol (PTMEG)as initiator, boron-trifuoride ethylether complex(BF₃·OEt₂)as catalyst, allyl glycidyl ether(AGE)as monomer. The structure and properties of the copolyether were characterized by IR, ¹H NMR, ¹³C NMR, GPC and DSC. Results indicate that the chain segment ratio of PTMEG and PAGE in the synthesized copolyether is basically consistent with the feed ratio, the relative molecular mass of the copolyether is controllable and the molecular mass distribution is relatively narrow. The glass transition temperature of the copolyether is -80.3 ℃, the viscosity is1.94 Pa·s, The tensile strength and elongation at break at room temperature of elastomer film obtained after curing with tetramethyl-p-phenylnitril oxide(TTNO)are 1.2 MPa and 83%, respectively.