Carboxymethyl Acid-thinned Starch(CMAS) was prepared through acid hydrolyzation and carboxymethylation.The influences of reaction parameters on the degree of substitution(DS) and reactive efficiency(RE) were investigated.The effects of pH and salt concentration on viscosity of CMAS were discussed by FTIR,XRD,SEM and TG of CMAS techniques.The result shows that a maximum DS can be achieved 0.8025 with RE 57.92%,and the reactive temperature is 50 ℃ for 3h in ethanol-water reaction medium,and the particle size of CMAS ranges from 5 to 10 μm.It is also found that the viscosity of CMAS decreases with the decreasing of pH and the increasing of salt concentration.Furthermore,when the salt concentration reaches to 0.6%,the viscosity of CMAS declines to 4.37 mPa·s,which declines by 46.31%.