The invention discloses an Al-Ti-B refining agent and a preparing method thereof, and belongs to the technical field of refining agents. The Al-Ti-B refining agent comprises components including, by mass percent, 8.0%-12.0% of Ti, 3.0%-5.0% of B and the balance Al. The Ti/B molar ratio is equal to 1/2. Phase compositions include alpha-Al and TiB2. According to the requirements, H3BO3, TiO2, aluminum powder and an aluminum ingot are weighed; according to the molar ratio of components of the refining agent, Ti/B=1/4, and Ti/TiO2=2/3; the purity of the aluminum ingot is 99.9%; TiO2 and H3BO3 areevenly mixed, and heating is conducted at 300 DEG C for 2 h; the heated powder and the heated aluminum powder are mixed to be arranged in a die and are pressed into blocks; the aluminum ingot is heated and melted and is covered with cryolite; and melt is subjected to temperature rise and pressed into the blocks, stirring is conducted for reaction, pressing of C2Cl6 is conducted for refining, and slagging-off and casting are conducted. The size of TiB2 prepared through low-cost raw materials is smaller than 1 micron.