The energetic copolyether of 3-nitratomethyl-3-methyloxetane and tetrahydrofuran was synthesized using boron trifluoride etherate (BF₃·Et₂O) and 1, 4-butanediol as an initiator system for the cationic ring-opening polymerization. The effects of reaction conditions were discussed, and the molecular weight could be controlled. The copolymer was characterized by IR, ¹H NMR and ¹³C NMR respectively. Meanwhile, the differential scanning calorimetry (DSC) show that the glass transition temperature of copolymer is -55.3 ℃, the peak temperature of decomposition is 214.1 ℃.