The invention discloses a double-fiber fine-grain type asphalt mixture and a preparation method thereof. The asphalt-aggregate ratio of the asphalt mixture is 5.5 to 6.0 percent; asphalt is modified asphalt in which a non-amine anti-stripping agent accounting for 0.3 percent of the mass of the asphalt is doped; the nominal maximum particle size of aggregates is 13.2 mm; the passing rate of a screen hole of 4.75 mm is 25 to 30 percent, and the passing rate of a screen hole of 0.075 mm is 9 to 11 percent; the filler-bitumen ratio is 1.5 to 2.0; the porosity is 3 to 5 percent; the voids in the mineral aggregates is not less than 15 percent; the asphalt saturation rate is 65 to 80 percent; a fiber stabilizer is prepared from basalt fiber and lignin fiber; the basalt fiber is white flocculent basalt fiber, and the lignin fiber is gray granular lignin fiber; the fiber is automatically added into a blending pot through special fiber adding equipment. The double-fiber fine-grain type asphalt mixture disclosed by the invention is applicable to new construction and maintenance projects of expressways in southern high-temperature rainy districts in summer, and can effectively improve the resistance to rutting and water damage of an asphalt pavement and prolong the service life of the pavement.