The molecular weights of tetrahydrofuran (THF) polymerization products by BF3-epich-lorohydiin (ECH) catalyst system at 0-2℃ monthly increase as the reaction proceeds in dry conditions. A small amount of water introduced into the reaction system will sharply increase the molecular weight at high conversions. The molecular weight of the product could be precisely controlled at the molar ratio of water and BF3 being around 1.23 with high yield above 60%. The hydroxyl end-group functionality of the product is close to 2 and the content of the ECH derived units in each chain containing about 26.5 THF derived units is around 1.3.