The invention relates to a method for preparing superfine kubonit continuous nanofiber, which is characterized in that the polymer fiber prepared by adopting the electrical spinning technique is taken as a template agent, then the surface of the template fiber is evenly coated with a boron source precursor by adopting a certain coating technique, and the superfine kubonit continuous nanofiber can be obtained after polymer template agent desorption and azotization treating processes. The appearance of the kubonit fiber product can be controlled through adjusting relative preparation parameters, the diameter of the fiber can be as thin as 50 nm, and no carbon impurity is contained in the kubonit fiber product, so that the purity can be above 98%. The method has the advantages of simple process and cheap raw material and equipment, and the obtained kubonit continuous nanofiber has the advantages of superfine diameter, controllability, high purity and the like, so that the invention has good industrialization prospect.