Abstract: In order to study on dynamic fractural behavior of brittle material with crack defect under impact loading, the test system of digital laser dynamic caustics and the self-designed drop-hammer impact testing platform are used, failure modes and damage procedures of PMMA specimens with different crack defects under impact loading are recorded, dynamic stress intensity factor and velocity of running cracks during propagating are analyzed. The results show that crack defects have guiding effects on propagations of running cracks. The influences of crack defects on running cracks are different for various inclination angles. Thereinto, oblique crack defect and horizontal crack defect have apparent inhibiting effects on the propagations of running cracks.