A reflective fiber optic sensor that consists of one transmitting optical fiber and six receiving optical fibers with an inclined surface is developed to realize the online, nondestructive monitoring of the growth processes of moldson the surfaces of wooden cultural relics. A theoretical model of this sensor detection measurement is established, and the influences of the sensor structural parameters on its sensitivity are numerically simulated. The growth processes of Penicillium citrinum and Trichoderma longibrachiatum on the surfaces of wooden cultural relics are monitored online by the sensor. Results indicate that the sensor can accurately obtain the characteristic absorption peaks of Penicillium citrinum and Trichoderma longibrachiatum, which are 261 nm and 272 nm, respectively. The corresponding detection sensitivities are 6.49×10^-4 AU/μm and 6.85×10^-4 AU/μm (AU is a unit of absorbance), respectively. A good linear relationship is found between the sensor's output signal and the mold height. In addition, the growth of molds on the surfaces of wooden cultural relics can be inhibited via treatment by a sodium chloride solution with an 8% relative mass fraction. The proposed sensor can accurately identify the mold growing on the surfaces of wooden samples, and it has application prospect in the field of mold control for wooden cultural relics.