<p>Polymethyl methacrylate (PMMA), a transparent polymer material, is widely applied in aerospace, optical components, and protective structures owing to its excellent optical and mechanical properties. However, under complex service conditions, internal defects such as microcracks and pores can easily cause stress concentration and lead to fracture failure. To investigate the dynamic crack propagation behavior of PMMA specimens containing prefabricated defects under impact loading, a combined experimental system integrating drop-weight impact loading with digital laser dynamic caustics was established. This system, coupled with a high-speed camera, enabled real-time visualization and quantitative analysis of the caustic evolution near the crack tip. The effects of defect morphology and position on crack initiation, propagation path, dynamic stress intensity factor, and crack propagation velocity were systematically examined by introducing semi-circular defects with different configurations and offset distances. The results show that the crack propagation velocity and stress intensity factor of defect-containing specimens are lower than those of defect-free specimens, indicating a significant inhibitory effect of defects on crack growth. When the offset distance is small, the defect exerts an attractive effect on the crack propagation path, and the influence of the left-opening semi-circular defect is stronger than that of the right-opening one. As the offset distance increases, the influence of defects on crack deflection and energy release gradually diminishes. This study elucidates the regulatory mechanism of the geometric characteristics of prefabricated defects on the dynamic fracture behavior of transparent polymers and provides theoretical and experimental insights for the crack-prevention design and impact-resistant optimization of polymer structures.</p>