The invention belongs to the technical field of nano-fiber materials, in particular to a pure carbon fiber material adopting a 'vesical string' structure and a preparation method of the pure carbon fiber material. The preparation method comprises the following steps: preparing a spinning solution from a spinnable polymer material, and preparing nanofibers of uniform structures with an electrostatic spinning device; uploading iron oxide hydroxide cambiform nanorods on the surfaces of the nanofibers uniformly via a water bath or hydrothermal method; soaking an iron oxide hydroxide-modified fiber membrane in a dopamine solution, and controlling the thickness of a polydopamine coating layer by adjusting the concentration of the dopamine solution and the reaction time; carrying out high-temperature carbonization treatment, so as to realize fiber carbonization, and conversion of iron oxide hydroxide to ferroferric oxide as well as polydopamine to a N-doped carbon material; removing ferroferric oxide via soaking in an acid solution. The preparation method is safe and environmentally friendly; the prepared pure carbon fiber material is high in nitrogen content, specific surface area and electric conductivity, and stable in physical and chemical properties, so as to be an ideal electrode material for production of new energy devices such as a supercapacitor.