The invention discloses a preparation method of a starch/micro-nano cellulose masterbatch. The preparation method comprises the following steps: carrying out kneading oxidation on cellulose so as to oxidize hydroxyl groups on a cellulose molecule chain into carboxyl anions and weaken the interaction between cellulose molecules; preparing the cellulose into a pre-dispersed solution, carrying out mechanical splitting and enabling cellulose fibers to realize micro nanocrystallization; finally, mixing the cellulose fibers subjected to the micro nanocrystallization with starch and enabling a mixture to pass through a twin-screw extruder for extruding and granulating to obtain masterbatch which is a solid aggregate or is further processed into powder, wherein the content of micro-nano celluloseis 20 to 40 weight percent, the diameter of the cellulose is 300 nanometers to 8 microns and the content of the carboxyl on the cellulose is 2.5 to 3.8 weight percent. The preparation method disclosedby the invention has the advantages of novel idea, simple operation and easiness in realization; in addition, an irreversible fiber keratinization phenomenon caused by hydrogen bond blocking in the drying process of the micro-nanocellulose is avoided; the obtained masterbatch is convenient to store and transport, so that the cost can be greatly reduced; the masterbatch and starch slurry are mixedand are boiled into slurry for preparing starch/micro-nanocellulose composite slurry with good mechanical properties.