The invention discloses a preparation method of a flame-retardant composition based on titanium dioxide. The method comprises the following steps of 1, putting hydrophobic solid particles into air drying equipment to carry out full air drying; 2, putting the hydrophobic solid particles into a ball mill for ball milling; 3, putting the foaming agent, the stabilizer and the hydrophobic solid particles into a stirrer for mixing and stirring, putting solute into clear water to be mixed to form an aqueous solution, and adding the foaming agent, the stabilizer and the hydrophobic solid particles into the aqueous solution to form a fire extinguishing agent; and 4, putting the obtained fire extinguishing agent into a pressure-resistant container, and putting the fire extinguishing agent into the pressure-resistant container at 1.8-2.3 atmospheric pressure to store the fire extinguishing agent. According to the method, ball milling the hydrophobic solid particles is added, the unit diameter ofthe hydrophobic solid particles is obviously reduced, and can be more uniformly attached to the surface of a fire object, the hydrophobic solid particles cooperate with foam generated by a foaming agent, in a high-temperature burning environment, a layer of isolated film can be rapidly formed, and rapid fire extinguishing can be realized.