The invention discloses a halogen-free flame retardant polycarbonate alloy resistant to alternating cold and heat and a preparation method thereof; the alloy comprises: 50-80 parts of polycarbonate, and 10-10 parts of polysiloxane-polycarbonate copolymer 40 parts, 2-15 parts of modified polyurethane, 0.1-3 parts of flame retardant, 2-10 parts of toughening agent, 0.5-1 part of anti-drip agent, 0.3-2 parts of antioxidant, 0.2-0.5 part of lubricant , 0.2‑1 part of UV absorber. The alloy has excellent flame retardant properties, and the flame retardant level of the 1.6mm thickness spline reaches the UL94‑V0 level. At the same time, polysiloxane-polycarbonate copolymer and modified polyurethane are added to the conventional polycarbonate raw materials, which not only ensures good processing fluidity, but also helps to reduce the internal stress of the material and improve the material's performance under alternating cold and heat conditions. It can be used in construction, industry, electronic and electrical and other occasions with high requirements for cold resistance, heat resistance and flame retardant, and has a wide range of market application prospects.