Abstract: Aiming to adapt the marketable demand for high holding water and soft gel, carboxymethylation was used to improve the properties of κ-carrageenan. Based on single factor experiments, the parameters of carboxymethylation were optimized using water separating proportion and gel strength as indexes. The mechanism of modification was discussed by FTIR and NMR. The optimized reaction conditions were:the amount of sodium chloroacetate was 0.02 mol, the temperature of carboxymethylation reaction was 65℃, and the concentration of ethanol was 69%. Under the conditions, the water separating proportion and gel strength of modified carrageenan was 0.16% and 205 g/cm², respectively. The properties of holding water and soft gel were better, compared to the un-modified carrageenan, with water separating proportion of 4.60% and gel strength of 2120 g/cm². FTIR and NMR results showed that, carboxymethyl functional group was introduced into gel structure and the structure was not destroyed, so the gel strength decreased significantly while the water rate was reduced effectively.