The invention discloses a modified white carbon black and a preparation method thereof, belonging to the technical field of non-metallic inorganic material. The silane coupling agent modification treatment is firstly carried out on the white carbon black, then stearic acid solid is put into acetone solution to be dissolved; according to the mass ratio, the adding quantity of stearic acid is 1:1-2:1 of coupling agent; the white carbon black modified by the silane coupling agent is added into the mixed solution, and the modified white carbon black powder is obtained after being dried; the surface of the modified white carbon black powder contains CH3(CH2)16-CONH-(CH2)-Si-O-Si radical. The modified white carbon black prepared by the technical proposal of the invention has even particle size and high white content; meanwhile, as the surface of the modified white carbon black contains CH3(CH2)16 radical, hydrophobic oleophylic property of the white carbon black that is modified for a second time is good. The invention uses the coupling agent and the stearic acid which are low in price and has simple preparation technique. Therefore, the modified white carbon black prepared by the method has low cost and is favorable to large-scale popularization and application.