Cellulose nanocrystals (CNCs) have been widely used in various industries; however, green, efficient, low-cost and recyclable CNC preparation still faces great challenges. Herein, ferric chloride (FeCl3) solution was used as the solvent to develop a green and recyclable preparation system. Owing to the acidic environment provided by FeCl3, which is a Lewis acid, and the promote hydrolysis properties of Fe3+, the amorphous area of cellulose could be rapidly hydrolyzed in the hydrothermal environment and produce CNC. More importantly, the used FeCl3 solution could be reuse after simple separation. During the entire cycle, the solvent concentration and pH only slightly changed. From the results of SEM, the prepared CNC showed a “rice-like” shape with typical CNC dimensions (diameter was ~38 nm and length was ~440 nm). The yield was ~92%, which was much higher than the previously reported methods of CNC. Furthermore, those CNC also hold excellent thermal stability (Tmax= ~350 °C). More importantly, the dispersing performance and large specific surface area of CNC were studied by using it as a supporter of PPy. Owing to the residual Fe3+, Py could polymerization uniformly on the surface of CNC. Overall, a green  and economical way to achieve sustainable mass production of CNC was provided by this work.