Abstract: Polylactic acid (PLA) is a new green friendly material and has very promising applications. In this work, for effectively resolving the problems of poor toughness and low crystallization rate of PLA, a method of modifying PLA with cellulose was proposed. First, the BC-g-PLA grafting product was obtained by in situ ring opening of L-propyl cross-ester (LLA) using bacterial cellulose (BC) as the substrate, and then the grafting product was added to PLA as a toughening agent, and the composite film material was prepared by the solution casting method. The results show that the reaction efficiency of the solution grafting method is higher than that of the melt grafting method, and the grafting rate can reach 76.60%. Structural testing of the grafted products by FTIR, nuclear magnetic resonance spectrometer and XRD reveal that PLA is successfully grafted onto the BC surface. Polarizing microscopy observed that the spheres have the highest degree of homogeneous refinement when the loading of BC-g-PLA filler as a heterogeneous nucleating agent is 0.6%. It is found through mechanical property tests that the elongation at the break of PLA film can be increased by 175% and tensile strength by 22.7% after toughening and modification. The crystalline properties of the composite film material were tested by differential scanning calorimetry. The crystallinity increases from 2.53% unmodified to 13.26%, and the crystallization rate also increases.