A method for preparing chitosan nanoparticles in water phase is provided. The method comprises the following steps: (a) providing a chitosan solution having a concentration of about 0.05 w/v % to about 1 w/v %, (b) adding water first and then followed by acetic anhydride to the chitosan solution to carry out acetylation, wherein the concentration of acetic anhydride is from about 10 v/v % to about 30 v/v % of the total volume of the whole solution, and (c) subjecting the solution from step (b) to physical dispersion.