The reaction of cotton fiber and other polysaccharides with formaldehyde vapor under the catalysis of inorganic salts was studied It was found that the catalysts kind, concentration and reaction time played an important role in the vapor phase formylation The results indicated that ZnCl 2 was the most efficient catalysis among the salts ZnCl 2, Zr（SO 4） 2 and Al 2（SO 4） 3 There existed an optimum concentration when ZnCl 2 was employed, i e , the highest weight percent gain （ WPG ） was obtained at a catalyst content of 8 2×10 5 Meanwhile, extending the reaction duration was helpful to increase WPG Fourier transform infrared spectrometry （FT IR） and high performance liquid chromatography （HPLC） analyses illustrated that cross linking reaction happened between formaldehyde and fiber WPG was less when compared cotton fiber with other cellulose model compounds, which suggested the cross linking was related with steric effect and finite zone HPLC spectrum of cellobiose proved digestible polysaccharide formation