Enzyme-assisted aqueous extraction (EAEP) is an environmental-friendly method which is an alternative to conventional hexane oil extraction. During EAEP of soybeans, an additional by-product, residue fraction was produced. About 4.4kg residue would be produced for every liter soybean oil extracted. The residue fraction of EAEP of soybeans contains substantial amount of fibers, which can be considered as a rich resource of dietary fiber. A soybean dietary fiber was prepared from EAEP residues and further modified through carboxymethyl substitution. According to the results of single factor experiments, Box-Behnken response surface methodology was conducted to investigate the correlation between operating parameters and degree of substitution (DS). The results indicated that the effects of four parameters on DS followed an order of etherification time, chloroacetic acid dosage, alkalization temperature, and etherification temperature. For optimizing the four independent variables, square regression analysis was performed. The regression model was fitted with determination coefficient R2 of 0.9684 and optimal parameters were as follows: alkalization temperature was 25℃;dosage of chloroacetic acid was 1.05 times of the mass of raw fiber;etherification temperature was 70℃ and etherification time was 3.8h. The DS under above conditions was 0.4305. Compared with the raw sample, the water holding capacity, oil holding capacity and expansion capacity of modified fiber were 8.21g/g, 3.80g/g and 9.88mL/g, which were improved by 89.61%, 20.63% and 114.32%, respectively. The infrared spectrum showed that carboxymethyl substitution was occurred during modification. The modified dietary fiber from EAEP of soybeans exhibited desired functional properties, which can be used in meat, beverage and bakery products.