A nanocrystalline cellulose preparation method includes steps: using an acetic acid-sodium acetate buffer solution with pH being 4.8 to prepare a cellulose solution with the concentration being 0.03-0.05g/mL, adding cellulase for enzymolysis, and performing ultrasonic treatment at intervals of 12h in the enzymolysis process, wherein the concentration of the cellulose in the solution is 10-15U/mL, and ultrasonic treatment lasts 30-60min each time; after reaction is finished, taking out a sample, separating centrifugally, centrifugally washing precipitates, freezing and drying to obtain nanocrystalline cellulose. The one-dimension rodlike nanocrystalline cellulose is prepared by combination of cellulose endonucleases and the ultrasonic physical method. The nanocrystalline cellulose preparation method has advantages of safety, avoidance of pollution, high efficiency and low energy consumption; the nanocrystalline cellulose prepared according to the method has the advantages of high yield, high crystallinity, high thermostability and small particle size.