A Mach-Zehnder-structured CO sensor based on Co-tetrakis(4-carboxy- phenyl) porphyrin (Co-TCPP) metal-organic framework is proposed in this study. In the sensing structure, a thick taper is fused between two sections of thin-core optical fiber and between the thin-core fiber and the single-mode fiber, and Co-TCPP is coated on the thin-core fiber's surface. The nanosheet-like morphology and porous microstructure of Co-TCPP make the sensor not only easy to adhere to the fiber but also have a strong adsorption capacity for CO. The structure, morphology, and properties of the sensitive material Co-TCPP are characterized and analyzed by X-ray diffraction, scanning electron microscopy, infrared spectroscopy, and thermogravimetric analysis. The findings demonstrate that the sensor's sensitivity to CO mass fraction is 0.0497 dB/10^-6, the fitting degree to mass fraction-light intensity is as high as 0.99148, and the response time is around 120 s when the CO mass fraction is 40 × 10^-6, and it has good selectivity and time stability to CO.