In this paper, polyitaconic acid butanediol ester copolymer (PBI) was synthesized and then blended with polylactic acid (PLLA). The PLLA/PBI blends were prepared by adding different proportions of PBI. The crystallization properties of the films were measured by Wide angle X-ray diffractometer and Differential Scanning Calorimeter. The results showed that the addition of PBI promoted the crystal transformation of PLLA, and the crystallization rate of PLLA was accelerated. Tensile testing machine was used to test the mechanical properties of the film. Compared with PLLA film, the flexibility of PLLA/PBI film was greatly improved. When the amount of PBI was 15%, the elongation at break of PLLA/PBI film reaches 371.8%.When the crystallinity of the film reacheds 33.9%, the film still showed good flexibility. After isothermal crystallization of PLLA/PBI, the strength of PLLA/PBI film increased and the film exhibited good flexibility. When the content of PBI was 20%, the elongation at break of PLLA can still reach more than 50%.