Abstract: The synthesis of polyoxytetramethylene glycol by cationic ringopening polymerization of tetrahydrofuran was studied in use of a binary catalyst system of fuming sulfuric acid and cocatalyst B. The results indicated that the proper content of free SO3 in fuming sulfuric acid in ringopening polymerization were about 30%. The optimal conditions of polymerization were obtained through orthogonal experiments. The average molecular weight of polyoxytetramethylene glycol could be flexibly controlled by varying the amount of cocatalyst B. The unreacted tetrahydrofuran could be recycled after suitable treatments.