We investigated the faint fluorescence spectrum of carbon dioxide in the near-ultraviolet and visible regions using an intense relativistic electron beam accelerator with an energy of 0.2-0.3 MeV. Monte Carlo simulations were carried out in advance to maximize the signal-to-noise ratio and a self-normalization pulse experimental layout was designed to overcome the shot-to-shot fluctuations in different pulses of the accelerator. Ultimately, the relative proportion sequences in the near-ultraviolet and visible regions of a carbon dioxide fluorescence spectrum were successfully obtained for the first time. The result shows that there exists distinct difference between fluorescence spectrum and Cerenkov spectrum, which can therefore provide basic support for selectively reducing the unwanted below-threshold fluorescence response of a carbon dioxide gas Cerenkov medium.