The yield of copolymerization of tetrahydrofuran and propylene oxide (THF：PO =5--15，by wt.) using a binary catalyst of fuming sulfuric acid (21％ or 28％) and perchlorie acid is around 50一—60％，and the loss of THF in the reaction is below 10一15％． The average molecular weight of the product can be controlled in the range of 1000--2000 by varying the binary catalyst system．In difference with the usual product of copolymerization initiated by BF3一diol or SbCl5-diol system，the yield of copolymerization with the low PO feed is not decreased，the hydroxyl functionality is equal to 2，the end-groups are predominantly primary hydroxyls (around 65--70％) and the product is colorless．