Abstract: The dynamic behavior of two notched boreholes in PMMA material with three prefabricated cracks under blasting load was studied by a new digital laser experimental system of dynamic caustics and the influence of crack prefabricated position on crack propagation and fracture failure was analyzed. The experimental results show that the stress intensity factor of the proximal prefabricated crack of the borehole increases first and then decreases after the two boreholes detonate at the same time. The explosion load is divided into two stages:loading stage and unloading stage. In the unloading stage,the stress intensity factor of crack tip decreases gradually,but the expansion speed is larger,this stage is dominant during the explosion. The stress intensity factor of the intermediate prefabricated crack which exten- sion effect is obvious is gradually reduced from the peak of 6. 52×105 N / mm3 / 2 . In the intermediate prefabricated crack tip of the two sets of samples,the extension direction of the crack tip is different,indicating that the intermediate pre- fabricated crack extension belongs to the unloading extension. The explosion stress wave determines the direction of the intermediate prefabricated crack tip,and the detonation gas plays a leading role in the process of intermediate prefabricated crack. The extension effect of the intermediate prefabricated crack is affected by the vertical distance of the two secondary cracks passed through the intermediate prefabricated crack,the smaller the distance,the more fully the in- termediate prefabricated crack tip expands.