Abstract: A new grafting product CDA-g-PET was synthesized using polyethylene terephthalate (PET) with low molecular weight and cellulose diacetate (CDA) as raw materials, isophorone diisocyanate (IPDI) as grafting agent and dibutyltin dilaurate (DBTDL), 1-butyl-3-methylimidazolium chloride as grafting promoter. The grafting process was optimized via exploring the type of catalyst and the amount of grafting agent. The mechanism of grafting was explored by Fourier infrared spectroscopy and wide angle X-Ray Diffraction (XRD) analysis. Furthermore, the mechanical property of CDA-g-PET film was influenced by the content of PET. The results show that PET is grafted onto CDA by chemical bond linking, and reduces the crystalline of CDA. Grafting time decreases to 8 h from 7 d due to DBTDL and ionic liquids as combined catalyst. When the amount of grafting agent for n(IPDI)/n(PET) is 1.05~1.10, there is no crosslinked CDA-g-PET found. With the increase of PET content, the tensile strength of CDA-g-PET film decreases by 6.55%~27.24% and the elongation at break increases by 149.89%~177.81% compared with CDA film.