The invention relates to a high molecular polycarbonate alloy material and a preparation method thereof. The alloy material comprises the following raw materials in parts by weight: 60 to 80 parts of PC, 20 to 30 parts of ABS, 1 to 4 parts of compatibilizer a, 1 to 4 parts of compatibilizer b, and 0.2 to 0.4 part of antioxidant, wherein the compatibilizer a and compatibilizer b are maleic anhydride grafted polyethylene, and the softening point of the polyethylene for grafting is not higher than 85 DEG C. The preparation method comprises the following steps: at first, blending PC and the compatilizer a once, extruding the mixture to obtain PC/PE-g-MAH, then fully mixing PC/PE-g-MAH with ABS, the compatilizer b, and an antioxidant in a high speed mixing machine, extruding the mixture by a dual screw extruder, and finally carrying out secondary melting and granulation to obtain the high molecular polycarbonate alloy material. The prepared polycarbonate alloy has a stable property and an excellent performance on resisting impact. The phenomenon of severe layering and cracks on the surface during the processing process are improved.