In this paper, cellulose nanocrystal (NCC) was acetylated modified by acetic acid, perchloric acid and acetic anhydride. Then polylactic acid/modified NCC (mNCC) composite materials were prepared by solution casting. The transparency and light transmittance of composite materials were studied through UV visible spectrophotometer. The morphology of mNCC in composite materials was observed by FE-SEM electron microscopy. Dynamic thermodynamic analysis was carried out by Dynamic Mechanical Analyzer. The results showed that compared to NCC, mNCC had better dispersion in PLA matrices when the content of mNcc was less than 4%. The nano-particles tended to aggregate with further increase of mNcc content. The storage modulus of composite materials was improved with the increasing mNCC content and the existence of mNCC nearly had no influence on glass transition temperature. This study provides certain experimental and theoretical reference for the improvement of the processing performance of PLA.