The invention relates to a preparing technology of metal composite coating, in particular to a coating preparing method and device by mixing metal and plastic powders. The aerodynamic spraying technology is adopted in the invention: metal and plastic powders are mixed; part of compressed air which carries powder when passes a powder feeder and gas preheated by a heater is mixed at an air inlet of a supersonic flow nozzle and then the mixed gas passes the nozzle to develop a gas-solid two-phase flow; solid particles in the gas-solid flow are sprayed on the surface of a workpiece and then develop serious plastic deformation and deposit on surface of the workpiece; and subsequent high kinetic energy particles repeat the above process to form alloy or coating. The method has the advantages of simple structure, low cost and high efficiency and can be applied to composite coating preparation. The device is provided with a high-pressure air source, the heater, the powder feeder, and the supersonic flow nozzle, wherein the high-pressure air source is connected with an air inlet pipe; and the air inlet pipe is connected with the supersonic flow nozzle through the powder feeder and the heater respectively. The device has the advantages of simple structure and strong practicability.