The invention discloses a preparation method of modified sodium alginate electrospun nanofibers, belonging to the field of natural biopolymer materials. The preparation method comprises the steps of: firstly, reacting sodium alginate with octylamine in the presence of EDC-HCl (1-thyl-3-(3-dimethyllaminopropyl) carbodiimide hydrochloride) as a catalyst to prepare modified sodium alginate, then dispersing the prepared modified sodium alginate powder into water at a certain concentration to prepare a modified sodium alginate spinning solution with a certain concentration, adding the spinning solution into an injector, regulating voltage to 10-20KV, setting the distance between a sprayer and a receiver to be 10-20cm and setting the flow to 0.3-0.8ml/h, and starting a spinning device. At present, sodium alginate electrospun nanofibers are mainly prepared by wet process spinning, and the wet process spinning needs complicated post-treatment such as crosslinking, washing and drying; and according to the preparation method provided by the invention, the nanofibers with better hydrophobic property can be directly obtained.