In karst areas, train vibration load has become an important factor that causes surface karst collapse around railway. In order to study the dynamic displacement and stress response of train load to tunnel, ground and stratum when a new subway tunnel is excavated in karst stratum, taking Guiyang Metro Line 3 crossing Sicuan-Guizhou Railway as the background, the finite element software modeling is used to calculate under the most unfavorable conditions of surrounding rock and train dynamic load. The dynamic response rules of three conditions of no cave above the tunnel, with cave and cave grouting are compared and analyzed. The results show that: Under the action of train dynamic load, the influence range of the karst cave and grouting reinforcement on the surface displacement dynamic response is roughly within 3.3 times the diameter of the karst cave. Moreover, when there is a karst cave under the railway subgrade, the maximum instantaneous settlement of the surface occurs at the intersection between the middle line of the subgrade and the middle line of the tunnel, which is 2.74mm. After grouting reinforcement of the karst cave, the settlement here is 2.16mm，decreased by 21.2 percent. In addition, the maximum instantaneous displacement and principal stress, which are -1.41mm and -0.36MPa, occur in the tunnel vault. When there are karst caves in the stratum, the vertical stress attenuation from the surface to the tunnel vault is the most obvious, from -0.0766 MPa to -0.0084 MPa, with a attenuation rate of 89.03%. It can be seen that the ground instantaneous settlement caused by train dynamic load can be significantly improved and reduced under the condition of ensuring the safety of the new tunnel after grouting reinforcement of the karst cave between the railway and the tunnel.