Poly (butylene itaconate) copolymer (PBI) was added to the main chains of poly (L-lactic acid) (PLLA) and poly (D-lactic acid) (PDLA) respectively, resulting in the synthesis of poly (lactic acid-butylene itaconate) copolymers, PLBI and PDBI, with equal molecular weight. The copolymers were blended into PLBI/PDBI films with varying mass ratios. As the blending ratio of PLBI and PDBI increases, the total crystallinity (Xtotal) first rises and then falls. PLBI/PDBI50% film can form a completely stereocomplex crystal (SC) structure, with an Xtotal of 32.44%, a melting point of 189 ℃, and a remarkable improvement in thermal properties. The elongation at break of PLBI/PDBI blended films first decreases and then increases, reaching the maximum of 185%, which is 14 times higher than that at a similar tensile strength. The tensile strength of PLBI/PDBI50% is 36.5 MPa, and the Young’s modulus is 1014 MPa, indicating brittle fracture. The barrier properties first improve and then deteriorate, with the O2 transmittance of PLBI/PDBI50% film dropping to 102 cm3/(m2·d), and the UV transmittance dropping to 11.39%. The O2 transmittance and UV transmittance are both 50% lower than those of the PLBI or PDBI films, which greatly enhance the barrier properties of PLLA.