Abstract: Abstract:Carboxymethyl starch (CMS) with high degree of substitution (DS) was synthesized from wheat starch and chloroacetic acid in the present of sodium hyroxide. The process conditions were optimized by orthogonal experiment with the amount of starch of 4 g, the optimized conditions were obtained as follows：amount of water of 6 mL，amount of absolute ethyl alcohol of 60 mL，molar ratio of chloroacetic acid to starch 1.4，molar ratio of sodium hydroxide to chloroacetic acid 1.8，amount of alkalization of sodium hydroxide 70%，alkalization temperature 35℃～40℃，alkalization time 0.5～1 h，etherification temperature 50～55℃，etherification time 2～3 h，the value of DS was 1.21 under the synthetic conditions.