The invention relates to the technical field of building materials, and concretely relates to a slag micropowder type artificial marble and a preparation method thereof. The slag micropowder type artificial marble comprises, by weight, 100 parts of unsaturated polyester resin, 0.5-2 parts of a peroxide initiator, 0.1-2 parts of an accelerant, 5-3 parts of a vinyl resin diluent, 5-30 parts of a cross-linking agent, 100-700 parts of slag micropowder, 0.5-3 parts of a coupling agent, 5-30 parts of a fire retardant 1, 50-100 parts of a fire retardant 2, and 0.01-0.1 parts of a nanometer siliver antiseptic. The compressive strength, the bending strength and the oxygen index of the slag micropowder type artificial marble are above 140MPa, above 32MPa and above 32 respectively, and the slag micropowder type artificial marble has a substantial fire retardation effect, and allows the use amount of magnesium hydroxide to be 50% lower than the use amount of single use of a magnesium hydroxide fire retardant, so the raw material cost of the marble is greatly reduced. The artificial marble also has a substantial bacteriostasis effect.