The invention belongs to the technical field of waste incineration, particularly relating to an air inlet device of a cement decomposing furnace. A furnace body of the cement decomposing furnace in the invention is provided with an air supply pipe from a flue gas outlet of a garbage gasification combustion furnace, a pipe orifice at the intersection between the air supply pipe and a furnace body of the cement decomposing furnace is in the same horizontal plane with a raw material inlet on the cement decomposing furnace. As the pipe orifice through which the garbage combustible gasified material enter the decomposing furnace and the raw material inlet on the cement decomposing furnace are in the same horizontal plane, so the combustible gasified material and the raw material as well as a high temperature material in the decomposing furnace can be thoroughly mixed; and due to a higher working temperature of a decomposing furnace in a cement pre-decomposition kiln, ash content in the garbage can be thoroughly decomposed, and the removal rate of the harmful components in the garbage through combustion can reach 99.99%, thus complete treatment of the garbage combustible gasified material containing the harmful components in the decomposing furnace of the cement pre-decomposition kiln can be ensured.