The invention relates to bis(tetrazolyl) boric acid ionic liquid and a preparation method thereof and belongs to the field of organic synthesis and spontaneous combustion liquid propellants. The bis(tetrazolyl) boric acid ionic liquid is prepared through replacement reaction between bis(tetrazolyl) sodium borate and corresponding imidazole, pyrrolidine or pyridine halogeno salt in an organic solvent; furthermore, the bis(tetrazolyl) boric acid ionic liquid has very high nitrogen content, positive enthalpy formation, high density, wide liquid range, lower viscosity, good thermal stability, acceptable ignition delay time and potential application value in the field of spontaneous combustion liquid propellants. The invention further provides a preparation method of safe bis(tetrazolyl) sodium borate. The method can stably control system temperature and meanwhile reduces danger in a stirring process by adding an organic solvent.