The invention discloses a method for preparing nanocellulose microfibrils through pretreatment of a ternary eutectic solvent, which belongs to the technical field of biomass nanocellulose preparation. The method comprises the following steps of firstly, pretreating lignocellulose biomass by using a ternary eutectic solvent, and separating to obtain a component rich in cellulose, preparing a 1wt% solution from a cellulose-rich component, centrifuging to obtain a nanocellulose suspension, carrying out high-strength ultrasonic treatment to fibrillate nanofibers, and freeze-drying to obtain the nanocellulose microfibril. The particle size range of the prepared nanocellulose microfibril is 26.84-61.26 nm, the removal rate of lignin reaches 56.28%, the effect is remarkable, operation is easy, the condition is mild, the cost is low, and pollution is small; and meanwhile, the used eutectic solvent is very environment-friendly, meets the standard of green chemical production, and has a relatively good practical application value.