The invention belongs to the technical field of pavement materials, discloses an activated carbon fiber based composite photocatalyst preparation method and solves the problem that conventional TiO2 only absorbs ultraviolet light and is high in surface energy, prone to agglomeration and difficult in gathering of pollutants on the TiO2 surface. The method includes: pretreating activated carbon fibers, adding methyl alcohol into a reactor, sequentially adding tetrabutyl titanate, glycol ether, lanthanum nitrate solution, glucose solution, lauric acid and activated carbon fibers, well mixing, performing reaction for 24h in the reactor, cooling to the room temperature, and cleaning and drying with absolute ethyl alcohol to obtain an activated carbon fiber based composite photocatalyst; finally, assessing effectiveness of the composite photocatalyst in catalytic degradation of main components in tail gas under simulated sunlight, adjusting the concentration of lanthanum nitrate and glucosesolution to determine optimum lanthanum and carbon doping quantity in TiO2 to prepare the activated carbon fiber based composite photocatalyst. The activated carbon fiber based composite photocatalystprepared according to the method is capable of absorbing visible light, and pollutant gathering promotion and photocatalytic efficiency improvement are realized.