The present invention relates to a lactide copolymer having good flexibility in addition to excellent properties such as mechanical properties and processability and preferably being applicable as a packaging material, a preparation method thereof, and a resin composition including the same. The lactide copolymer includes two or more specific block copolymerized repeating units that the hard segments of polylactide repeating units are connected to both ends of the soft segments of polyether polyol repeating unit, and the block copolymerized repeating units are connected to each other through the intermediation of urethane connecting groups derived from polyvalent isocyanate compound of which the average equivalent of isocyanate group per a molecule is more than 2 and less than 3.