Endogenous carbon monoxide (CO) is an essential cell signaling molecule, which is closely related to numerous physiological and pathological processes. Therefore, it is of great significance to monitor CO in living samples. Fluorescent probe technique provides an effective and convenient method for monitoring CO. Although many fluorescent probes of CO have been reported, most of them require the introduction of heavy metal ions (Pd2+), which is not conducive to the practical application of these probes. Herein, a metal-free coumarin-based fluorescent probe was developed for monitoring CO. For this probe, coumarin was selected as the fluorophore, the aromatic nitro group was used as the reactive site, and the carboxyl moiety acted as the water-soluble unit. As a result, this probe has been proved to be able to monitor CO with good specificity and excellent sensitivity in water medium. After interacting with CO, the aromatic group was reduced to an aryl amino group; accordingly, the emission intensity of the peak corresponding to the probe at 450 nm significantly increased. Moreover, it successfully realized the detection of CO in living cells by a fluorescence signal.