Abstract: The present invention relates to a transparent copolyester, wherein the transparent copolyester comprises an aliphatic-aromatic copolyester segment A, a segment B having repeating units —O—CH(CH3)—C(O)—, and structural units C derived from polyisocyanate(s), wherein the weight ratio for the segment A, segment B and structural unit C is 100:(100-2000):(0.1-10) and wherein the weight-average molecular weight Mw of the transparent copolyester is from 50,000 to 1,000,000. The present invention further relates to a preparation method for a transparent copolyester, including polymerizing lactide, a hydroxyl-terminated aliphatic-aromatic copolyester and a polyisocyanate in the presence of a catalyst; wherein the weight ratio for the aliphatic-aromatic copolyester, lactide and polyisocyanate is 100:(100-2000):(0.1-10). The present invention further relates to a transparent copolyester prepared by said method and an article made from the transparent copolyester according to present invention.