Abstract: Corn fiber was modified by twin- screw extrusion technology.The extruded melt temperature, water addition and rotary speed of screw were optimized through single factor and orthogonal experiment according to the yield of soluble polysaccharides.The reducing power and scavenging effects of corn fiber soluble polysaccharides on the hydroxy free radical, superoxide anion radical and DPPH · were determined by Phenanthroline-Fe2 +method, pyrogallol autoxidation method and colorimetric methods.The results showed that the optimum conditions were as follows, rotary speed of screw 150r /min, the melt temperature 140℃ and the water addition 20%.Under this condition, the yield of soluble polysaccharides was 2.7 times higher than before, reached to 24.91%.The purity of crude polysaccharides had improved and the molecular weight had been decreased.The reducing power and scavenging effects of treated soluble polysaccharides on the hydroxy free radical, superoxide anion radical and DPPH·were improved greatly compared with untreated ones.