In order to study the shear strength of fiber-water soluble polymer composite and the failure mechanism of sand, the shear strength characteristics of improved sand under different polymer contents and fiber contents were studied through a series of direct shear tests and numerical tests.The shear strength, parameters and failure characteristics of improved sand were analyzed. The test results show that the shear strength characteristics of the improved sand increase significantly with the polymer content and fiber content. The incorporation of water-soluble polymers and fibers increases the cohesion and has little fluence on the internal friction angle of the improved sand. When the fiber content is 0.8% and the water-soluble polymer content increases from 1% to 4%, the shear strength of the improved sand increases by 55.58.kPa. When the water-soluble polymer content is 4% and the fiber content increases from 0.2% to 0.8%, the cohesion increases by 32.69.kPa. The increase in polymer and fiber contents promotes the bonding between the components of the composite sandy soil, enhances the overall integrity of the improved sandy soil, and improves the stability of the soil.