The invention discloses a preparing method for a sodium alginate nanofiber based on a self-assembly technology. According to the preparing method, sodium alginate molecules are self-assembled into a nanofiber structure by utilizing induction of ethyl alcohol. The preparing method comprises the steps of firstly using deionized water to prepare a 0.01wt%-30wt% sodium alginate solution; then, stirring while slowly adding dropwisely an absolute ethyl alcohol solution into the sodium alginate solution until the mass of absolute ethyl alcohol accounts for 5%-90% of the total mass of the mixed solution; finally, still standing the obtained mixed solution for 2-120 hours to make the sodium alginate molecules slef-assembly into the sodium alginate nanofiber. The preparing method for sodium alginate nanofiber based on the self-assembly technology is environment-friendly in technology, short in process, simple, easy to control, and simple in adopted instrument/equipment; the shape and the size of the sodium alginate nanofiber can be regulated and controlled by changing assembly conditions, the product is good in quality and convenient in long-term storage, and applicable to basic raw materials of pharmaceutical gels, textile fibers, pharmaceutic preparations and the like in many fields.