Taking cassava starch as the raw materials and hydrochloric acid (HCL) as the catalyst, the acidolysis modification of starch was performed combining with the size mixture blending process. The results show that when the concentration of cassava starch is in the range of 8%~12%, the dosage of 6% hydrochloride acid for 1000g starch milk is about 0.8g~1.2g, and the viscosity of acidolysis starch decreases with the increase of hydrochloric acid dosage. The viscosity can reach the stable level when the reaction time is about 2 hours for the size mixture of low viscosity, and is about 3 hours for that of the high viscosity. Also, the viscosity thermal stability of size mixture is verified to be good.