Polytetramethylene glycol (PTMG) was prepared by the ring-opening polymerization of tetrahydrofuran using a binary catalyst of 21% fuming sulfuric acid and perchloric acid with the once-through yield about 60% and the ultimate yield around 90-98%. The molecular weight of PTMG was controlled in the range of 1000-4500 by varying the binary catalyst system.The recovered tetrahydrofuran can be reused for polymerization after distilling with a short column and drying by sodium hydroxide.