With sweet potato residue as raw material, cellulose nanocrystals (CNCs) were achieved through ultrasonic-assisted acid method. Carboxymethyl cellulose nanocrystals (N-CMCs) were produced by carboxymethylation of CNCs. Transmission electron microscopy (TEM) showed that the N-CMCs were still typical spherical nano particles and slightly larger with a uniform diameter ranging from 30 nm to 50 nm. They were more loosely packed than the CNCs and had a porous surface structure. The crystallinity index of the N-CMCs (52.83%) was lower than that of CNCs (73.27%), whereas the thermal stability of the former was higher. N-CMCs could be used as an excellent food additive, with low viscosity less than 25 mPa瘙簚s, high purity more than 99.5% and nano-sized scale.