In order to explore the dynamic crack propagation law of beam members,bar defects with the heights of 22 mm,28.5 mm and 35 mm were designed in a three-point bending beam,and then tested by the digital laser dynamic caustic line experiment system and the drop weight impact test system.The results show that the crack propagation rate and stress intensity factor are affected by the defect height in two stages in the drop weight impact experiment.In the first stage,the maximum growth rates of crack propagation have a trend of first rising and then stabilizing as 247.49 m/s,292.49 m/s and 284.99 m/s with the increase of defect height,and the cracking stress intensity factor was respectively 1.480 MPa/m3/2,1.665 MPa/m3/2 and 1.812 MPa/m3/2,which increased with the increase of defect height.Meanwhile,the crack initiation and propagation rates respectively decreased by 634.42 m/s,524.97 m/s and 377.67 m/s,the cracking stress intensity factors of KⅠ type respectively decreased by 3.281 MPa/m3/2,3.192 MPa/m3/2 and 2.876 MPa/m3/2,and the cracking stress intensity factors of KⅡ type respectively increased by 1.254 MPa/m3/2,1.319 MPa/m3/2,and 1.398 MPa/m3/2.Furthermore,the KⅠ-KⅡ composite stress intensity factor was transformed into a KⅠ type stress intensity factor by deflection.