A method for preparing surface-modified nano silicon dioxide from rice hulls. The method includes: 1) pretreating rice hulls using a treating gas containing CO2 to remove metal ions, impurities, and dusts, and desiccating and grinding the rice hulls; 2) submerging the rice hulls into a dilute solution of phosphoric acid, boric acid, hydrochloric acid, formic acid, acetic acid, propionic acid, butyric acid, or a strong-acid-weak-base salt for between 4 and 8 hrs, controlling the immersion temperature not to exceed 10° C., leaching a resulting mixture, removing a filtrate, and desiccating the rice hulls; and 3) calcining the rice hulls in the absence of oxygen at a temperature of between 300 and 450° C.