The disclosure discloses a preparation method of a composite coating for a resin matrix composite, comprising the following steps: preparing ceramic-resin composite powders which comprise Al2O3 ceramic, a thermosetting resin and a curing agent and are semi-thermosetting resin powders; and respectively spraying pure Al2O3 ceramic powders and the composite powders on the surface of the resin matrix composite by supersonic atmospheric plasma spraying to form a ceramic-resin composite coating, wherein the pure Al2O3 ceramic powders are fed into jet flow in a manner of feeding powder inside a spray gun, and the composite powders are fed into jet flow in a manner of feeding powder outside the spray gun. Correspondingly, the disclosure also provides a preparation device of a composite coating for a resin matrix composite.