The invention provides an asphalt pavement gap sealing and filling material and a preparation method thereof, and belongs to the technical field of road building materials. The asphalt pavement gap sealing and filling material comprises the components in percentage by weight: 70 to 90 percent of SBS (styrene-butadiene-styrene) modified asphalt and 10 to 30 percent of composite modified diatomite, wherein in the SBS modified asphalt, the SBS copolymer mixing amount is 3 to 5 percent and the stabilizer mixing amount is 0.1 to 0.3 percent; the composite modified diatomite is prepared from diatomite, rock asphalt, rubber and polyethylene according to parts by weight. The preparation method comprises the steps of weighing, heating and mixing the diatomite, the rock asphalt and the rubber powder according to a ratio, adding the polyethylene and grinding the polyethylene into powder, and mixing uniformly to obtain the composite diatomite; heating the base material asphalt, adding the SBS copolymer, heating to 170 to 180 DEG C, shearing after stirring, adding the stabilizer, developing for 1 to 2 hours, adding the composite diatomite and stirring to obtain the sealing and filling material. The asphalt pavement gap sealing and filling material and the preparation method thereof have the advantages of high impact resistance, high adhesive strength, high weather resistance, simple process and the like, and can be widely applied to asphalt pavement crack treatment.