The invention discloses a method for producing high-viscosity and high-degree-of-substitution carboxymethyl cassava starch by utilizing an ultrahigh-pressure blasting technique. The method comprises the steps such as alkaline treatment, etherification reaction and purification. The method is carried out by primarily achieving the purpose of destroying a crystallization region of cassava starch particles by utilizing sodium hydroxide, sufficiently swelling and alkalifying starch molecules and loosening starch particles under the ultrahigh pressure of a high-pressure homogenizer, so that the hydroxyl of the starch becomes negative oxygen ions and the nucleophilic ability is strengthened, and simultaneously generating an St-O-Na<+> active center, then, preparing the high-viscosity and high-degree-of-substitution carboxymethyl cassava starch through the bimolecular nucleophilic substitution between sodium carboxymethyl starch obtained by alkalifying the starch and chloroacetic acid under the alkaline condition of sodium hydroxide. By utilizing the high-degree destructive effect of the ultrahigh-pressure blasting technique on a crystallization structure of the cassava starch, the technical problems such as degradation of the starch particles in a production process of the high-viscosity and high-degree-of-substitution carboxymethyl cassava starch are solved.