The invention provides a preparation process of transition metal boride, comprising the following steps: A) aluminum is put in a reactor, inert gas is fed into the reactor after evacuation, the reactor is heated up to 700 to 800° C. and then added with dry potassium fluoborate or sodium fluoborate, monomer boron and cryolite are generated by rapid stirring and reaction for 4 to 6 hours, and the molten liquid at the upper layer is sucked out and the monomer boron is obtained by means of separation; and B) the obtained monomer boron is added with transition metal for reaction at the temperature from 1800 to 2200° C. in order to generate corresponding transition metal boride.