The present invention relates to a polylactic acid resin composition which has a low glass transition temperature, fusion temperature, and enthalpy of fusion, can be crystallized under commercially meaningful processing conditions, has good film processability such as extrusion properties, has excellent storage stability, and is highly biodegradable. The polylactic acid resin composition comprises a block copolymer comprising a hard segment and a soft segment, the hard segment comprising polylactic acid repeat units, and the soft segment comprising polyurethane polyol repeat units which have polyether-based polyol repeat units linearly connected via a urethane bond, wherein the soft segment is contained by 5% to 35% by weight based on the weight of the block copolymer, and the polylactic acid repeat units include poly(L-lactic acid) repeat units and poly(D-lactic acid) repeat units by a molar ratio of 94:6 to 88:12.