The experiment used defatted rice bran as materials which extract water-soluble polysaccharide from it, expecting to improve its functional activities by modification. The carboxymethylated derivative of polysaccharide was prepared by the reaction between rice bran polysaccharide and chloroacetic acid (MCA) in alkaline condition and fourier infrared spectrum suggested that carboxymethylated rice bran polysaccharide was successfully carboxymethylated without changing the original structure of rice bran polysaccharide. The optimal technological conditions for the reaction are: reaction temperature 52 ℃, reaction time 2.5 h, concentration of NaOH 1.2 mol/L, MCA dose 3.20 g. This condition of the rice bran polysaccharide carboxymethylated substitution degree(DS) resulted to a maximum value of 0.99.