The invention discloses a reinforced flame-retardant low-warpage PBT alloy material and a preparation method thereof. The alloy material comprises the following components in parts by weight: 35-50 parts of polybutylene terephthalate, 5-15 parts of polycarbonate, 15-30 parts of glass fiber, 5-15 parts of an inorganic filler, 5-20 parts of a compound flame retardant, 5-10 parts of a toughening agent, 0.2-0.4 parts of a compound antioxidant, 0.1-0.3 parts of a lubricant, 0.1-0.3 parts of an ester exchange inhibitor, and 0.1-0.3 parts of an anti-dripping agent. The preparation method comprises the following steps: uniformly mixing all the materials except the glass fiber at a high speed, respectively feeding the mixed material and the glass fiber into a main feeding hole and a side feeding hole of a double-screw extruder, and finally forming the material. The amorphous polymer PC is introduced into the raw materials, so that the crystallinity of the system is reduced, the warping propertyof the PBT material is improved, and the PBT can be reinforced and toughened; and a compound flame retardant is introduced, so that the product has a flame-retardant characteristic. Finally, the PBT/PC alloy material has good mechanical properties, flame retardance and warping resistance, so that the application range of the PBT/PC alloy material in the fields of high-grade electrical components,household electrical shells and the like is expanded.