Ion mobility spectrometry (IMS) has become the most deployed technique for on-site detection of trace explosives, and the reactant ions generated in the ionization source are tightly related to the performances of IMS. Combination of multiform reactant ions would provide more information and is in favor of correct identification of explosives. Fast switchable CO3–(H2O)n and O2–(H2O)n reactant ions were realized in a dopant-assisted negative photoionization ion mobility spectrometer (DANP-IMS). The switching could be achieved in less than 2 s by simply changing the gas flow direction. Up to 88% of the total reactant ions were CO3–(H2O)n in the bidirectional mode, and 89% of that were O2–(H2O)n in the unidirectional mode. The characteristics of combination of CO3–(H2O)n and O2–(H2O)n were demonstrated by the detection of explosives, including 2,4,6-trinitrotoluene (TNT), cyclo-1,3,5-trimethylene-2,4,6-trinitramine (RDX), ammonium nitrate fuel oil (ANFO), and black powder (BP). For TNT, RDX, and BP, product ions with different reduced mobility values (K0) were observed with CO3–(H2O)n and O2–(H2O)n, respectively, which is a benefit for the accurate identification. For ANFO, the same product ions with K0 of 2.07 cm2 V–1 s–1 were generated, but improved peak-to-peak resolution as well as sensitivity were achieved with CO3–(H2O)n. Moreover, an improved peak-to-peak resolution was also obtained for BP with CO3–(H2O)n, while the better sensitivity was obtained with O2–(H2O)n.