The present invention relates to a lactide copolymer having excellent general physical properties such as mechanical physical properties, processability, and the like, and having excellent flexibility to be effectively usable as a packaging material, and the like, a preparing method thereof, and a resin composition including the same, wherein the lactide copolymer includes: two or more block copolymer repeating units in which hard segments of polylactide repeating units are bound to both ends of soft segments of polyether polyol repeating units, the block copolymer repeating units are linked with each other via a urethane linking group induced from a polyvalent isocyanate compound having more than 2 to less than 3 equivalents of an isocyanate group per a molecule, and the polyether polyol repeating unit has a number average molecular weight of 1000 to 10000.