To investigate the value of fine-grained soils applicable in roadbed engineering through large-scale dynamic triaxial tests. The effects of fine-grained content and reinforcement structures on subgrade soil dynamics under cyclic loading were systematically examined. Sand-clay mixtures with varying fine-grained contents were tested to analyze axial cumulative deformation，dynamic stress-strain hysteresis curves，and elastic modulus. Results indicate that axial deformation is first decreased then increased with rising fine-grained content，reaching a minimum at 20%，while the most rapid growth in dynamic elastic modulus is observed at this critical threshold. Fluctuating upward trends are exhibited by the stress-strain hysteresis curves of all specimens，with specimens containing 20% fines being found to enter the elastic deformation stage earliest. Compared to unreinforced soils，axial deformation is reduced by 53% after 5 000 cycles through double-layer composite geogrid reinforcement，demonstrating enhanced effectiveness at higher vibration cycles. However，deformation resistance is found to weaken as reinforcement layers are increased. Significant improvement in deformation resistance of fine-grained soils is achieved through reinforcement. Superior performance in constraining soil movement and improving dynamic strength is demonstrated by composite geogrids compared to other materials，attributed to enhanced frictional interaction. Nevertheless，fine-grained content is determined to exert greater influence on soil performance than reinforcement measures. Critical insights for optimizing subgrade reinforcement design in transportation engineering are provided by these findings，emphasizing the importance of balancing material selection with fine-grained content control for enhanced long-term stability.