Abstract: In order to study the effect of different shape defects on the dynamic fracture of brittle materials under impact load, choose PMMA (polymethyl methacrylate) as the experimental object, drop hammer as the loading source, at the same time, the length of the defect in the loading direction of falling weight is kept unchanged, using a new type of digital laser dynamic caustics experimental system, carry out the impact three-point bending test on PMMA specimens.At the same time, analyses the failure mode, time history characteristics, dynamic stress intensity factor and crack growth rate of the specimens.The experimental results show that except for the square defect (two stress concentration points) specimens, the fracture of other defect shape specimens is mainly mode I.The specimen with one stress concentration point is the first to be damaged, the second is the specimen with two stress concentration points, and the last one is the specimen without stress concentration point.In the control group of specimen with one stress concentration point, the energy convergence time of triangular defect specimen is shorter than that of vertical defect specimen, the energy convergence time has a positive correlation on the crack initiation speed.It can be concluded that:the shape of the incision at the initiation end has no significant effect on the failure form of the plate and the dynamic mechanical properties of the non-incision periphery, but the initiation time and peak velocity of the plate will change by affecting the dynamic strength factor at crack initiation of the plate.