The invention discloses copolyester which comprises the following components in percentage by weight: 30%-94% of a crystalline aromatic polyester unit A and 6%-70% of a lactone unit B, wherein the molar ratio of the lactone unit B to the crystalline aromatic polyester unit A in the copolyester is 40%-90%. The condensation polymerization temperature is reduced to be 220-239 DEG C in a reaction process, so that the production energy consumption is reduced; in addition, a high-activity titanate catalyst is selected and used, so that the reaction rate of the lactone unit B is increased to be 40%-90%, the content of the lactone unit in distillates of a vacuum system is reduced, the distillates can be conveniently separated, and the production cost is lowered.