Abstract: The fiber reinforced soil has good application prospects, and its stress-strain constitutive model is the basis of engineering analysis and design. A two-phase constitutive model for the fiber-reinforced soil is developed considering the fiber-reinforced soil as a kind of composite materials comprising of basic phase and fiber phase using the modified Cambridge model and the linear elastic model respectively. A reasonable strength model is introduced as the failure criterion for the fiber-reinforced soil, and fiber reinforcement weakening at large strain is also taken into account. At the same time, consolidated drained triaxial shear tests on reinforced sand with fiber volume fraction of 1% are performed. The two-phase constitutive model is validated, and the results are in good agreement with the experimental data.