The invention relates to a polycarbonate / polyamide alloy and a preparation method thereof. The alloy contains the following components by weight percentage: 30-80 of polycarbonate, 20-70 of polyamide, 5-30 of toughening agent, 1.5-15 of compatibilizer and 0.2-3 of assistant; wherein the contents of the toughening agent, the compatibilizer and the assistant are based on the total weight of the polycarbonate and the polyamide. Compared with the prior art, due to the blending of the polycarbonate and the polyamide with the excellent compatibilizer in the preparation method, the preparation process is reasonable, the prepared polymer has good paint adhesion, solvent resistance and impact resistance and simultaneously combines the low moisture absorption, heat resistance, wear resistance and self-lubricity of the polycarbonate, and the polycarbonate / polyamide alloy can be widely applied in automobiles (front and rear baffles, column covers, wheel covers and instrument covers), electronic appliances (electric tool housings, wiring devices and relays) and other fields.