Heavy haul railway transportation usually enhances freight volume by increasing axle load of the train. However, this change always produces a considerable cyclic loading on coarse-grained soil filling in subgrade. It may result in excessive plastic deformation or even subgrade destruction. These subgrade defects mostly relate to the dynamic failure rules of coarse-grained soil filling. This paper focuses on the dynamic failure rules of subgrade filling. A series of large-scale dynamic triaxial tests is carried out. The influence of confining pressure and water content on plastic strain increase is analyzed. All the specimens are classified as stability type or failure type according to their features of plastic strain increment. An empirical formula of critical dynamic stress is also established by the classification. Moreover, the possibility of subgrade destroy under different axle loads is discussed. Finally, the rules that dynamic strength changes with confining pressure and water content are investigated. The correlation of static strength and dynamic strength is verified. An empirical formula for estimating dynamic strength according to static strength is proposed by the normalizing data.