Hexylated carboxymethyl starch with high viscosity was prepared by esterification of hexanoyl chloride and carboxymethyl starch using dimethyl sulfoxide (DMSO) as solvent and pyridine as catalyst. The reaction conditions, such as the reaction temperature, time, solvent volume and the amount of catalyst were investigated and the optimum reaction conditions were obtained as follows: 5 g CMS/40 mL DMSO (0.125 g/mL), reaction temperature 50 ℃, reaction time 30 min, organic base 1.0 g（0.025 g/mL）. Hexylated carboxymethyl starch was characterized by IR and the viscosity properties of solution were investigated using Brookfield R/S - CC rheometer. Result of viscosity properties of different concentration and DShexyl of HCMS indicates that the viscosity value of HCMS can increase to 34000 mPa.s（γ= 5 s-1）at DShexyl of 0.3 when the concentration reaches critical aggregation concentration(2.0% w/w).