The invention discloses a diatomite/nylon-6 composite material and a preparation method thereof, and belongs to the field of plastic filling modification. The composite material is composed of 70 to 95% of nylon-6 and 5 to 30% of diatomite; wherein the particle size of diatomite is 5 to 40 [mu]m, and the diatomite is processed by a surfactant. The preparation method comprises the following steps: drying diatomite, processing the dried diatomite by a surfactant through a dry-process technology, wherein the surfactant accounts for 0.2 to 1 wt% of the weight of diatomite; pre-mixing nylon-6 and processed diatomite according to a certain formula, adding the premix into a double screw extruder, mixing, cooling by water, and granulating to obtain the composite material. The nylon-6 is modified by filling diatomite, the strength, modulus, toughness, and thermal deformation temperature of nylon-6 are all enhanced, the cost is reduced, the using amount of nylon-6 is reduced, the energy is saved, and the environment is protected. Moreover, the application range of diatomite is enlarged, and the utilization value of diatomite is increased.