In this paper, the experimental research was made for the desalination and purification Chiitooligosaccharides (COS) solution with nano-filtration (NF). The optimal operation conditions of NF purified COS solution were obtained as follow: 35~40℃, 1.0MPa operation pressure, COS solution diluted 2.5 times to be the feed solution of NF, and the optimal solution volume concentration factor of a period of the batch constant volume diafiltration process was 1.5. Under this conditions, the monosaccharide and most of disaccharide and monovalent salts were removed from COS solution by NF, and the purity of high biological activity COS was up to 92%, at the same time the volume was condensed to half of the initial feed solution.