To improve the rapid decline in the strength of a certain section of saline soil in the vicinity of Qinghai Lake after freeze-thaw cycles, two types of materials were used: sulphoaluminate cement (SAC) and SAC with the optimal mixing ratio combined with different amounts of sisal fiber (SF) through unconfined compressive strength (UCS) tests and direct shear tests to compare the improvement effects of different modified materials on saline soil under freeze-thaw cycles. The X-ray diffractometer (XRD) and scanning electron microscope (SEM) were used to analyze the influence of the modified materials on the microstructure of the saline soil. The results showed that both SAC and the composite material could significantly enhance the mechanical property parameters of the saline soil. The composite material with the optimal mixing ratio (0.7% SF + 10% SAC) had significantly better mechanical property parameters than the single 10% SAC modified soil, and its mechanical property parameter loss rate was the smallest under freeze-thaw conditions. The UCS and cohesion of the soil were negatively correlated with the number of freeze-thaw cycles. The internal friction angle of the soil after SF combination was more obvious in the freeze-thaw process than that of SAC modified soil. The hydration products of SAC caused the strain type of the soil to change to strain softening, and SF effectively reduced the brittleness index. The freeze-thaw cycles led to the development of the microscopic structure of the saline soil in a direction unfavorable to the soil strength. The structural damage degree of the soil after freeze-thaw of SAC modified soil was relatively limited, while the physical restraint formed by the bridging effect and the synergistic action of the hydration products in the SF bridging effect make the composite material-modified soil exhibit stronger frost resistance.