A reflective fiber-optic spectral sensor is developed to realize online nondestructive monitoring of the growth process of Aspergillus versicolor on the surfaces of paper cultural relics. The sensor consists of one tapered input and six receiving fibers. First, the principle of sensor measurement of mold microorganisms on the surfaces of paper cultural relics is established. Then, the morphological characteristics of Aspergillus versicolor on the surfaces of paper samples are derived. Finally, the growth process of Aspergillus versicolor on the surfaces of paper samples is monitored online by the sensor. The results indicate that the sensor can accurately obtain the characteristic absorption peaks (295 nm and 390 nm) of Aspergillus versicolor. When the height of Aspergillus versicolor is in the range of 101.1‒596.0 μm, the output signal of the sensor has a linear relationship with the growth height of Aspergillus versicolor, and the detection limit reaches 10 μm.