Using a GDS hollow cylindrical torsional shear apparatus to model the stress path of strain-induced vibration, we investigated the dynamic strength of Nanjing fine sand under different loading conditions. To simplify the loading method, we modeled the relationship of the normal and shear stresses with an oval, rather than heart-shaped, stress path. Our results show that when the confining pressure and the amplitude of the train-induced vibration are small, the dynamic strength of the Nanjing fine sand first increases and then stays stable over time. However, when the confining pressure increases, the increasing stage of the dynamic strength of the Nanjing fine sand is very short and the dynamic strength values are also much smaller. After some vibration time, the dynamic strength of the Nanjing fine sand decreases almost linearly with increasing vibration. Also, as the amplitude of the vibration increases, the dynamic strength weakens more quickly. This also proves that the drain condition mainly affects the increasing stage of the dynamic strength and has very little influence on its weakening stage.