A fiber-optic carbon monoxide (CO)-sensing system with high sensitivity was proposed by combining Fabry–Perot interferometer (FPI) sensor with sensing material Au/ZnO. The sensitivity is 814  pm  /    %   within the CO concentration range of 0% to 4%. The FPI sensor was fabricated by inserting single-mode fiber into a partially polymer-filled glass capillary to form an air cavity; the length of the air cavity is 76.83  μm. The temperature sensitivity of the FPI is −2.7  nm  /    °  C, which provides better conditions for measurement of CO. The catalyst Au/ZnO was prepared by coprecipitation, which could catalyze the oxidation of CO at ambient temperature and humidity. X-ray diffraction and scanning electron microscope analysis were employed to reveal the crystalline structure and morphology of the catalyst. To eliminate the influence of ambient temperature change on the accuracy of the concentration test, a fiber Bragg grating with central wavelength of 1550.24 nm was introduced in the experimental setup for the temperature compensation.