The present invention relates to a method for preparing surface-functionalized cellulose nanospheres, which comprises (1) adding cellulosic fibers to the alkali liquor, swelling, filtering the solution, washing the product with water for several times until pH is 7, and drying to constant weight; (2) then adding the above product into mixed acid for fully infiltrating, reacting the above solution under stirring for 2-10 hours at 60-90 DEG C, naturally cooling the reaction products, adding an alkali solution until the pH of the solution is adjusted to 7 to obtain a dispersion liquid of cellulose nanospheres, then centrifuging the dispersion liquid, washing with water for three times, carrying out ultrasonic treatment, and then carrying out freeze drying to constant weight to obtain a cellulose nanospheres. The preparation method has advantages of low raw material cost, simple preparation process and mild preparation conditions; the surfaces of the cellulose nanospheres have functional groups, so that thermal stability and hydrophobicity are improved, self-aggregation is reduced, dispersion is easy, size is small, distribution is narrow, control is easy, specific surface area is large, reactivity is high and etc.