Fiber reinforcement is an effective method for improving engineering properties of soil. However, the interaction mechanism of the fiber and the surrounding soil is not well understood. Based on mechanical analysis of fiber-soil interface under pullout condition, a tri-linear model is proposed to describe the shear stress-displacement relationship. The progressive pullout process of a short fiber in soil is divided into five consecutive phases: (1) the initial pure elastic phase (Phase I); (2) the elastic-softening phase (Phase II); (3) the pure softening phase (Phase III); (4) the softening-residual phase (Phase IV); and (5) the final pure residual phase (Phase V). For each phase, the analytical solutions of the distributions of tensile force, interfacial shear stress and displacement are derived. Through a comparison between the pullout test results of polypropylene fiber (PP-fiber) and the predicted results, the effectiveness of the proposed model in capturing the progressive load-deformation behavior of a short fiber in soil is verified. Moreover, the effects of water content and dry density of soil on the model parameters are analyzed in detail. It is found that the interfacial peak/residual shear resistance and shear stiffness of fiber reinforced soil significantly depend on soil compaction conditions. In general, two transition phases (Phase II and Phase IV) are not evident during the whole pullout process of PP-fiber.