A series of laboratory pullout tests on the sensing optical fiber embedded in expansive soil were performed to study the impacts of three factors on soil-fiber interfacial mechanical properties, including dry density of soil, water content and overburden pressure. It is found that the displacement-pullout force curves under different test conditions have the similar rules. The pullout force increaces linearly with the pullout displacement to the maximum value, then decreases gradually and finally tends to the residual value. The optical-soil interfacial mechanical characteristics are analyzed based on a tri-linear pullout model. The shear strength of fiber soil interface is positively correlated with soil dry density and soil overburden pressure but is negatively related to soil moisture content. The study results will provide references for the further application of fiber optic sensing to expansive soil deformation monitoring in geotechnical engineering.