The invention relates to a super-tough PC (polycarbonate)/PBT (Polybutylece Terephthalate)/PET (Polyethylene Glycol Terephthalate) alloy and a preparation method thereof. The alloy comprises the following components in percentage by weight: 35-65% of PC, 10-40% of PBT, 10-40% of PET, 4-8% of toughening compatilizer, 0.4-1% of antioxidant and 0.4-1% of processing agent, wherein the toughening compatilizer is an ethylene-methyl acrylate copolymer and/or an ethylene-methyl acrylate-glycidyl methacrylate copolymer. The preparation method comprises the following steps of: drying raw materials; then sufficiently mixing; co-mixing; extruding and pelletizing. Compared with the PC/PBT alloy, the PC/PBT/PET ternary alloy has remarkable advantages in the aspects of rigidity, toughness, heat resistant temperature and size stability, has higher cost performance and greatly improved toughness, and is the alloy with excellent comprehensive performance and high cost performance.