The reinforcement mechanisms of polymer composites loaded with short fibrous fillers (SFF) were deeply analyzed. The recent progresses in this research field were reviewed, such as the quantitative characterization of the reinforcement for polymer/SFF composites, especially for the determination of the strength including the tensile strength, the young’s modulus and the interfacial strength between the fibers and the matrix. On the basis of the relevant literature and the previous studies by the author, the systematic and complete descriptions of the reinforcement mechanisms of polymer/SFF composites were proposed, including the interfacial adhesion reinforcing theory, the fiber skeleton stiffening theory, the fiber-induced crystallization stiffening theory (for crystalline matrix) and synthetic reinforcing effect theory.