The invention provides a gas injector and cover plate assembly comprising a cover plate, a gas injector and a ceiling. The cover plate comprises a plurality of cooling fluid channels. The gas injector is configured to be located on the cover plate comprising a gas distributor, a fluid-cooling gas transmitter, a plurality of gas spraying plates and a conducting cone. The gas distributor distributes a plurality of gases and a gas transmitter cooling fluid. The gas distributor comprises a gas conduit for introducing a first gas. The fluid-cooling gas transmitter connects the gas distributor to introduce the gas transmitter cooling fluid to form a plurality of cooling fluid walls and the first gas and the gases. The gas spraying plates and the conducting cone are located beneath the fluid-cooling gas transmitter.