Cellulose Nanocrystals (CNCs) are widely used in various industries; However, green, efficient, inexpensive and recyclable CNC machining still faces major challenges. Here ferric chloride (FeCl3) solution was used as a solvent to develop a green and recyclable manufacturing system. Due to the acidic environment of FeCl3, a Lewis acid, and the hydrolysis-promoting properties of Fe3 +, the amorphous area of cellulose could be hydrolyzed quickly in the hydrothermal environment and produce CNC. More importantly, the used FeCl3 solution can be reused after simple separation. Solvent concentration and pH changed only slightly throughout the cycle. From the results of the SEM, the prepared CNC showed a “rice-like” shape with typical CNC dimensions (diameter ~ 38 nm and length ~ 440 nm). The yield was ~ 92%, which was much higher than the previously reported methods of the CNC. In addition, these CNCs also have excellent thermal stability (Tmax = ~ 350 ° C). More importantly, the dispersing performance and large specific surface area of CNC have been studied using it as a supporter of PPy. Due to the remaining Fe3 +, Py was able to polymerize evenly on the CNC surface. Overall, this work provided a green and economical path to sustainable mass production of CNC.