The invention provides a modified nano microcrystalline cellulose composite chitosan fiber, and further provides a preparation method of the modified nano microcrystalline cellulose composite chitosanfiber. The modified nano microcrystalline cellulose composite chitosan fiber is composed of a chitosan fiber matrix and modified nano microcrystalline cellulose distributed in the fiber matrix, and the modified nano microcrystalline cellulose accounts for 3-10% of the mass fraction of the composite fiber. According to the preparation method of the modified nano microcrystalline cellulose composite chitosan fiber, the production can be realized by depending on a conventional chitosan fiber production line, the method is simple, and the cost is low. The composite chitosan fiber prepared by thepreparation method is entirely composed of chitosan and cellulose which both belong to polysaccharides and have good compatibility; and the ionic interaction exists between the modified nano microcrystalline cellulose and a chitosan molecular chain to enable a mixed solution to have a certain gelation phenomenon, the guarant viscosity is risen, multiple times of extension of the coagulation bath and the plasticizing bath can be withstood, and the mechanical properties of the composite fiber are effectively improved.