In order to improve the shear strength characteristics of sand, it is aimed at sisal fiber reinforced sand composite material in this paper. The effect of fiber content, fiber length and dry density on the shear characteristics of reinforced sand was thoroughly studied by unconsolidated-undrained triaxial shear test so as to investigate the reinforcement mechanism of fiber-reinforced sand. The test results show that fiber reinforcement has little effect on the initial stiffness of sand. Appropriate fiber content and fiber length can form a three-dimensional network structure in sandy soil, which higher than that of unreinforced samples. It shows that fiber reinforcement can effectively improve the shear strength of sand. The shear strength of the composite material increases significantly due to the increase in the bite and sliding friction between the fiber and sand particles in the shear process. The increase of confining pressure on the effective contact area between fiber and sand results in the increase of shear strength and peak deviatoric stress. Sisal fiber reinforced sand can enhance the interfacial force between sand and improve the shear strength of sand.