As shown in Equations (1)–(4), water molecules play a vital role in initiating the Cr3+-catalyzed hydrolysis process by generating H3O+ ions. In order to accelerate the rate of the hydrolysis reaction, it was suggested that water content be increased in order to produce more hydronium ions by shifting the reaction forward to the product side (Le Chatelier’s principle). Therefore, the effect of solid-liquid ratio of crystallinity index of nanocellulose was investigated by varying the ratio from 1:10 to 1:50 at a constant metal salt concentration (0.8 M), and 1.5 h reaction at 80 °C. As shown in Figure 1d, the nanocellulose yield increase when more water was added into the hydrolysis system (higher solid-liquid ratio); this might be similar to decreasing the metal salt concentration. In other words, the acidity of the reaction mixture was reduced by water molecules. However, the crystallinity of nanocellulose reached a maximum value (87.2%) at the solid-liquid ratio of 1:30 and gradually decreased when the cellulose matrix was treated with a more diluted hydrolysis system (1:40 and 1:50). This result might reflect that a small amount of H2O could reduce the cellulose hydrolysis efficiency due to the incompletion reaction between the cellulosic material and hydrolyzing catalyst. In addition, it was observed that some portion of the cellulosic material failed to immerse completely in the reaction solution when the solid-liquid ratio set at 1:10, which also leads to the hydrolysis process not taking place effectively. However, the catalyst concentration significantly reduced when a large amount of water was introduced to the reacting system, and this eventually led to a decrement of the crystallinity of nanocellulose as shown in Figure 1c. In fact, it is important to control the consumption of water in actual industrial production. Taking the cost and efficiency into consideration, 1:30 was chosen as a suitable loading ratio between cellulose solid material and the catalyst aqueous solution.