The invention relates to an ionization source, and specifically relates to a dual-zone chemical ionization source for mass spectrometry. According to the scheme of the invention, a reagent ion generating chamber and an ionization chamber of a to-be-tested sample are designed as a dual-zone structure, the reagent gas is ionized in the reagent ion generating chamber, reagent gas neutral molecules and metastable molecules are directly discharged from the chamber by a tail gas pipeline, and charged particles enter the ionization chamber of the to-be-tested sample through a transmission channel tointeract with to-be-tested sample molecules to form target ions, and enter a mass spectrometer under the action of a specific electric field to realize the detection. The unique feature and main advantage of the invention lie in the design of the dual-zone structure, the neutral molecules and metastable molecules cannot enter the ionization chamber of the to-be-tested sample, and a pure reaction environment can be maintained to avoid interference with the detection of the target ions.