In the ultra-broadband range, by continuously tuning the wavelength of the pump laser to excite the rubidium-helium (Rb–He) mixture, an obvious blue fluorescence signal can always be detected. Through the experimental study of the difference between resonant excitation and non-resonant excitation to produce blue fluorescence, it is verified that Rb–He excimer plays an important role in ultra-broadband absorption, and five mechanisms for producing blue fluorescence are summarized: (1) excimer + one-photon resonant excitation process from 5P; (2) two-photon resonant excitation process from 5S; (3) one-photon resonant excitation process from 5S + energy pooling process; (4) excimer + one-photon resonant excitation process from 4D; (5) excimer + energy pooling process. The influence of ionization on the generation of blue fluorescence is discussed. Under several typical pump wavelengths (based on different mechanisms of generating 420 nm fluorescence), the variation of 420 nm fluorescence with the Rb vapor atom concentration, the characteristics of 420 nm collimated light, and its competition with the fluorescence were discussed. The ultra-broadband absorption properties of the alkali metal-noble gas system can be applied to the detection of infrared signals.