Disclosed is a method for preparing polyester resins copolymerized with glycol for modification such as 1,4-cyclohexanedimethanol, in which the content of said glycol ingredient for modification ranges from 20 to 80 mol % of the total glycol content. Dicarboxylic acid such as terephthalic acid is esterified with ethylene glycol and said glycol for modification. The esterification product is subjected to polycondensation in the presence of a titanium dioxide/silicon dioxide coprecipitate catalyst, said catalyst being in the form of 4% or less strength by weight suspension in glycol. The copolyester prepared as above shows excellent transparency and color properties in addition to being produced at high efficiency.