Abstract: A composite nanofiltration membrane that includes a support layer having a thickness of 100 nm to 10 mm and a cross-linked chitosan thin layer having a thickness of 10 to 1000 nm, in which the support layer is formed of a polymer; the cross-linked chitosan thin layer, disposed only on the top surface of the support layer, contains links each formed from a cross-linking agent, which is trimesoyl chloride, iso-phthaloyl dichloride, sebacoyl chloride, or a combination thereof; and the membrane has a mean pore size less than 1 nm. Also disclosed are a method for preparing such a composite nanofiltration membrane and a composite nanofiltration membrane prepared according to the method.