Based on the pendulum impact experiment of the straight wall arch cavern, PFC^3D was used to simulate the failure process and analyze the meso-mechanical behavior including stress, crack, velocity evolution. The research results indicate: (1) Under the impact load with a frequency of 250 Hz, the development of cracks goes through three stages: initiation period, rapid growth period, and slow growth period. The growth rate of the number of cracks can reach as high as 10⁵ per second. The type of cracks is primarily tensile-shear mixed cracks dominated by tension. (2) The magnitude of the crack initiation stress is influenced by the shape of the cross-section. There are different stress response patterns at different locations, with the corner reaching the crack initiation stress firstly, followed by the arch shoulder. (3) During the crack development stage, the velocity vectors around the straight wall arch undergo a spatial distribution change from “center symmetry → upper-lower symmetry → left-right symmetry”, and the magnitude of the velocity vectors changes from “near-large and far-small”to “near-small and far-large” with the straight wall arch as the center. Cracks initate in areas where the magnitude of the velocity vectors changes significantly. The research findings of this paper can provide a reference for a deeper understanding of the failure process and mechanism of tunnels under the impact loads.