Abstract: To address potential safety hazards like microcrack initiation, deformation, and failure inside coal pillars, this study monitored the horizontal deformations of coal pillars using the Brillouin optical time-domain reflectometry (BOTDR)—a distributed optical fiber sensing technology. Firstly, this study established a method for converting the axial strain of optical fibers into horizontal deformations based on a theoretical analysis of the failure and bulking characteristics of coal pillars. Secondly, it determined the axial tensile response of metal-based cord-shaped sensing optical fibers by conducting laboratory tests. Then, it corrected the strain coefficient based on the deformation and failure patterns of coal pillars. Finally, against the engineering background of the Huojitu well of the Daliuta coal mine in Shaanxi Province, this study implemented on-site monitoring of the horizontal deformations of section coal pillars in mining face 22206. The key results are as follows: (1) Significant differences in horizontal deformations were observed between the mining and non-mining sides of section coal pillars. Specifically, the mining sides exhibited the largest horizontal deformation, about five times that of the non-mining sides. However, coal pillars manifested minor overall horizontal deformation characterized by only elastic deformation, suggesting relatively stable coal pillars. (2) Coal pillars displayed significant deformation zones, presenting an elastic core zone in the central part but plastic failure zones on both sides. (3) As the mining face advances from 20 m in front of the measurement ponit to 40 m behind the messurement point, the horizontal strain of coal pillars increased exponentially, and after moving away from survey points, the deformations tended to stabilize, reflecting a dynamic link between coal pillar deformations and mining-induced stress disturbance. (4) A analysis on the horizontal strain and vertical stress of coal pillars revealed an exponential relationship between both, forming a quantitative relationship between both. The result indicates that the deformations and failure of coal pillars largely depend on the distribution and evolution of mining-induced stress. Overall, the BOTDR-based monitoring of the deformations inside section coal pillars can yield accurate information on microdeformations inside section coal pillars, serving as an important guide for determining coal pillar sizes, controlling surrounding rocks, and evaluating pressure in goaves.