Abstract: In the blasting operation of underground projects,the existence of joints,fissures and other defects often plays a vital role in the final blasting effect,especially in the case of inclined defects,the phenomenon of over-excava- tion and under-excavation is very prominent. Because of the complexity of crack dynamic propagation in an inclined defect model,a new digital laser experimental system of dynamic caustics was used to study the propagation laws of blast-induced cracks in PMMA material with prefabricated fissure under the condition of simultaneous initiation for the two-hole blasting. The experimental results show that the blast-induced main crack and the blast-induced secondary crack propagate in opposite directions respectively,and the two cracks do not meet directly in the propagation process,but move to the near blast-induced crack surface in the curve way. In the early stage of crack propagation,the crack tip of explosive crack is affected by both compressive wave and shear wave,and the initiation time of the shear wave is lat- er than that of the compressive wave. The crack tip is mainly propagated by the tensile stress produced by the compres- sion wave. The stress intensity factor KI at the crack tip is larger than KII at the same time. The crack propagation ve- locity,stress intensity factor and energy release rate increase first and then decrease. In the later stage of crack propa- gation,the crack tip is mainly subjected to the explosive gas pressure,and the characteristic quantities change in an os- cillatory manner. In a certain range,with the increase of the inclination angle of prefabricated fissure,the peak value of characteristic quantities is higher,the propagation time of main crack and derived crack is longer,and the propagation distance is longer. With the increase of the inclination angle of prefabricated fissure,the ultimate failure area is larger, and the interaction between blasting load and prefabricated fissure is more obvious. The results of this experiment pro- vide a reference for analyzing the physical process of blasting crack propagation especially when there are joints,cracks and other defects.