Abstract: With the continuous increase in train speed standards, the vibration amplitude between wheels and tracks intensifies, resulting in greater loads transmitted to the subgrade. These elevated stresses accelerate the development of subgrade deterioration, thereby compromising operational safety, train speed, and passenger comfort. Stability of railway subgrade structures under higher-speed conditions has thus become a critical engineering challenge. The incorporation of waste tire fragments as backfill material offers dual benefits—enhancing geotechnical performance and promoting the sustainable reuse of solid waste materials while mitigating environmental pollution. However, limited studies have addressed the application of large-diameter tire fragments reinforced in railway subgrade soil and studying their dynamic stability. To verify the feasibility of using tire fragments as fill material for high-speed railway subgrade, a series of large-scale dynamic triaxial tests were conducted on mixed subgrade soil composed of natural subgrade soil and tire fragments, aiming to investigate the dynamic performance of the mixed railway subgrade soil. The results revealed that incorporating tire chips into natural subgrade soil can reduce axial cumulative deformation. The dynamic stress-strain hysteresis curve presents significant non-linear characteristics, and the addition of tire chips enhances the sensitivity of the sample to dynamic stress with an increase in dynamic load magnitude. The dynamic elastic modulus is relatively sensitive to the dynamic load magnitude, while the influences of confining pressure and the amount of tire chips on the development trend of the soil dynamic elastic modulus are not significant. Adding tire chips and increasing the confining pressure can improve the dynamic strength of sandy soil, with the optimal mixing ratio being 3.5%. These test findings offer valuable technical parameters and a theoretical foundation for analyzing the dynamic stability of railway subgrade filled with large-diameter tire fragments.