Abstract: The new testing system of digital laser dynamic caustics was developed to carry out four groups of millisec- ond blasting experiments for studing the dynamic mechanical behavior of explosive crack propagation with slotted car- tridge millisecond blasting. The results show that the longer the delay initiation time,the farther the propagation dis- tance of crack generated by the first detonating borehole,the higher the ratio of crack propagation distance before ac- tion of stress wave to the total distance,and the penetration point of two cracks gradually closes to the later detonating borehole,into a “ hand in hand” shape distribution;the compression effect from the positive pressure zone of stress wave and the stretching effect from negative pressure zone of stress wave can decrease and increase the velocity of op- posite crack propagation,respectively. The longer the delay initiation time,the lower the peak value of decreased veloc- ity of crack propagation and the higher the peak value of increased velocity of crack propagation. The compression stress field of the explosive stress wave at the opposite crack tip can decreases mode I dynamic stress intensity factor, and the longer the delay initiation time,the smaller its peak value;both the velocity of crack propagation and the dy- namic stress intensity factor increase suddenlly when two cracks meet. The earlier the encounter,the higher the peak value of velocity;when delay initiation time satisfies certain conditions,the stress wave generated by the first detonating borehole forms a tensile stress field near the borehole wall of the later detonating borehole,which is favorable to the in- itiation of crack.