Using the test system of digital laser dynamic caustics (DLDC), time-related fractural characteristic and crack tip stress change of PMMA specimens which contained bilateral off-center cracks were analyzed. The fracture toughness of specimens was about 1.16 MN/m3/2 under the loading condition. Intervals of the two bilateral off-center cracks almost had no effect on the impact fracture toughness. In a certain range, symmetric degree of crack propagation was improved while intervals of the two cracks increased. There was a mutational stress point of crack tips during main crack propagation. The degree of stress concentration was enhanced and failure mode was shifted from tensile failure to tensile-shear failure at the point. When the stress of secondary crack tips released, stress concentration of main crack was enhanced. Dynamic stress intensity factors of crack tips changed choppily during crack propagation. Compared to secondary crack tips, main crack tips kept advantage in stress all long.