A fiber-optic CO sensor based on the Ni-MOF-74 is proposed in this paper. Part of the Ni ions in the Ni-MOF-74 are replaced with Co ions by the doping method of ion replacement to generate Co/Ni-MOF-74. Co/NiO is then obtained by calcination. It has multiple active sites and a specific adsorption capacity for CO molecules. When the CO volume fraction is within 0--60×10 ^-6, the sensitivity of the fiber-optic sensing system integrating the Co/NiO can reach 60.58 pm/10 ^-6 and the detection limit is 179.76×10 ^-9. In addition, the sensor, with a high concentration-wavelength linear goodness-of-fit (fitting coefficient R²=0.97176) and favorable selectivity and temporal stability, has huge application potential for detecting CO at a low volume fraction.