The invention belongs to the technical field of high molecular materials, and particularly relates to halogen-free flame retardant high gloss polycarbonate (PC) / acrylonitrile butadiene styrene (ABS) alloy and a preparation method thereof. The alloy comprises PC resin, ABS resin, polymethyl methacrylate (PMMA) resin, composition flame retardants, antioxidants and composite surface brighteners. The method for preparing the alloy includes the following steps: weighing, by weight, the PC resin, the ABS resin, the PMMA resin, the antioxidants and the composite surface brighteners; putting the PC resin, the ABS resin, the PMMA resin, the antioxidants and the composite surface brighteners in a high-speed mixer for dry-blending; throwing the polymer blends into a twin-screw extruder for melt extrusion and granulation, wherein the composition flame retardants are added from a side feed port and the addition is controlled by a metering pump; baking the produced particles to obtain the halogen-free flame retardant high gloss PC / ABC alloy. The obtained halogen-free flame retardant high gloss PC / ABC alloy is high in gloss, heat resisting and fluidity, simple in technology, low in cost, and capable of being suitable for the industrial areas of machinery, automobiles, electronics, instruments and apparatus, textile, constructions and the like.