The work aims to use additive to improve the properties of cellulose (CE) in ionic liquid solvents. IL was used as CE solvent and flexible poly (PBS) was added to improve the toughness of CE film. The structure of composite film was studied. The mechanical property and stretching mechanism of composite film were researched by tensile tester and SEM. The results showed that when PBS content was 1%, the tensile strength and elongation at break of CE/PBS-1 were increased from 45.6 MPa and 7.21% to 58 MPa and 15.6% respectively, which were increased by 30.7% and 48.2% respectively. In conclusion, the PBS in the CE matrix is in the form of blending and no chemical bonding exists between them. The content of PBS significantly effects on the particle size and decentralized state. Lower content of PBS particles can be uniformly dispersed in CE matrix, which reduces the density of hydrogen bonds between the CE molecules, and is beneficial to the slip of molecular chain. Hence, the toughness of the composite film is enhanced.