The invention provides a microwave plasma ordinary-pressure desorption ionization source for generating plasma by applying microwave energy. The microwave plasma ordinary-pressure desorption ionization source comprises a novel microwave device capable of generating plasma, a microwave power source and a coaxial cable. The microwave device comprises an outer pipe, an inner pipe, a middle pipe and an adjusting piston, wherein the outer pipe, the middle pipe and the inner pipe are in an three-pipe coaxial structure and are respectively provided with two working gas inlets, one end of the coaxial cable is connected with the microwave power source, and the other end of the coaxial cable is coupled to the middle pipe. The ionization source can be combined with a plurality of mass spectrometers with atmosphere ports for use, can carry out desorption ionization on solid, liquid and gas samples under the ordinary-pressure environment without sample pretreatment, poisonous chemical reagents, high voltage and high-speed airflow, can shorten the detection time to the samples, can realize the real-time, on-line and non-destructive detection on the sample, can save the detection cost, cannot pollute samples and is environment-friendly; ions of obtained products are mostly molecular ions; and the obtained spectrogram is concise and distinct.