Basing on the dual demands of enhancing the shear bearing capacity of carbonate saline soil and improving the sustainable utilization of recycled tire polymer fiber (RTPF), a method of improving cement stabilized carbonate saline soil with RTPF was proposed. Through triaxial shear tests and microscopic inspections, the influence and mechanism of RTPF on the shear strength of stabilized carbonate saline soil were investigated. The results indicate that RTPF increases and then decreases the shear strength of stabilized carbonate saline soil. The optimal proportion is 1%~2% for fiber bundles and 0.1%~0.2% for fibrils. RTPF can improve the cohesion and internal friction angle of stabilized carbonate saline soil. The toughening effect of fiber bundle is higher than that of fibrils. The piecewise function can accurately describe the stress-strain relationship of RTPF stabilized carbonate saline soil.