The propagation behavior of fractures in natural rock masses under dynamic loading is critical for engineering safety. Based on the transmission dynamic caustic method, this study systematically investigates, for the first time, the influence of V-shaped pre-existing cracks with different opening directions (upward/downward) and angles (60°, 90°, 120°) on dynamic crack propagation in specimens under impact loading through three-point bending experiments. The results indicate that: (1) The crack opening direction is the key factor controlling the crack path: when the opening is downward, the propagation path remains similar to that of the defect-free specimen, resulting in a vertical and smooth fracture surface; when the opening is upward, the crack is “attracted” to the endpoint (P-point) of the pre-existing crack and then deflects to propagate along one of its arms, revealing a new mechanism where the coupling of opening direction and stress wave propagation direction influences path selection. (2) Time-history analysis shows that the total fracture time of specimens with upward-opening cracks is significantly prolonged, indicating an inhibitory effect of such defects on stress wave propagation. The crack propagation velocity peaks when passing the P-point, and the peak velocity decreases as the pre-existing crack angle increases. (3) The dynamic stress intensity factor exhibits a fluctuating trend of initial decrease followed by an increase during both crack initiation and propagation stages. However, the initiation toughness and maximum stress intensity factor are essentially consistent across all specimens, suggesting that the defects primarily affect the propagation process and path rather than the initiation threshold. This research provides important experimental basis for predicting the fracture behavior of rock masses containing complex defects under dynamic loads.