The invention relates to a polycarbonate alloy having high ultrasonic welding strength and a preparation method thereof. The polycarbonate alloy is prepared through the following steps: (a) weighing 20-50 parts of polycarbonate, 50-80 parts of ABS (Acrylonitrile Butadiene Styrene), 0.1-0.5 part of a welding strengthening additive, 0.1-1 part of an antioxidant, 0.3-0.6 part of a compatilizer in parts by weight, and mixing; and (b) feeding the mixture obtained in step (a) into a twin-screw extruder, fusing, extruding and pelleting to obtain the polycarbonate alloy. Compared with the prior art, the polycarbonate alloy has the advantages of high welding strength and no welding fragment, and is suitable for a relatively wide ultrasonic welding condition and ultrasonic welding for a dissimilar material piece, etc.