The invention relates to the technical field of fiber processing, in particular to a preparation method of banana nanofiber. The banana nanofiber is prepared according to the following three steps: (1), pretreating a raw material; (2), degumming; (3), preparing the nanofiber, wherein during degumming, puffing treatment, oxidation treatment, acid treatment, bioenzyme treatment, enzyme deactivationtreatment, dehydration, oil supply and drying are combined for pretreating crude banana hemp. Through organic combination of the treatment ways, colloid, lignin and hemicellulose in the crude banana hemp can be effectively removed, and the purposes of removing impurities and increasing the fiber content are achieved; a good raw material is provided for subsequent preparation of the nanofiber. Theinvention further prepares the banana nanofiber by an electrospinning technology. By the technology, the yield of the banana nanofiber can be effectively increased, the impurities can be effectively removed, the banana nanofiber with smaller fineness is produced, and thus the quality of the banana nanofiber is improved.