Cellulose nanocrystals (CNCs) were prepared by acid hydrolysis of cotton fibers, then modified using vinyl acetate and n-butylacrylate as monomer through radical polymerization. The modified CNC (m-CNC) and poly(lactic acid) (PLA) were compounded to prepare composites via solution casting technique. The structure and properties of m-CNC/PLA composites were analyzed by TEM, FT-IR, mechanical testing, SEM, WAXD and degradation test. The results show that rod-like CNCs obtained through acid hydrolysis have the diameter of 5 nm～26 nm and the length of 40 nm～380 nm. The tensile strength of m-CNC/PLA composites increases with increasing content of m-CNC. As the m-CNC content increases from 0% to 8%, the tensile strength increases by 43.3%. The PLA phase in m-CNC/PLA composites presents higher crystallinity but smaller crystallites compared with the pure PLA. Adding m-CNC into PLA slows down the degradation of m-CNC/PLA composite in simulated body fluid but accelerates its degradation in soil.