The invention discloses a preparation method of a hollow magnetic Co/carbon nano tube. The preparation method comprises the following steps of (1) weighing cyanoguanidine and cobalt acetate, adding cyanoguanidine and cobalt acetate to anhydrous alcohol, and magnetically stirring a solution for 2.5-3.5h at 58-62 DEG C, (2) stirring the solution for 22-26h at 58-62 DEG C, (3) drying the solution in a drying device, (4) roasting the solution for 0.5-1.5h at 850-950 DEG C in an N2 atmosphere in a high-temperature furnace at a heating rate of 9-11 DEG C/min, and (5) performing natural cooling to a room temperature to obtain the hollow magnetic Co/carbon nano tube. The invention further discloses an application of the hollow magnetic Co/carbon nano tube. The preparation method and the application of the hollow magnetic Co/carbon nano tube have the advantages that a preparation technology is simple, operation is convenient, the pollutant adsorption capacity is high, and separation from a water body in a later stage is convenient.