To investigate the dynamic propagation behavior and the dynamic fracture toughness of I/II mixed mode cracks, an improved large-size single cleavage semi-circle compression (SCSC) specimen made of sandstone is adopted in this study, and impact tests are carried out using a medium-low speed drop weight device. Crack propagation gauges (CPG) and strain gauges are used jointly to measure the crack initiation time and crack propagation velocity. Meanwhile, dynamic stress intensity factor is calculated based on the ABAQUS code . The results show that the crack propagation speed is not a constant, and the crack may arrest occurs during propagation. CPG can effectively characterize the crack propagation velocity and law. Furthermore, the AUTODYN software is employed to simulate crack propagation behavior, and the numerical simulation results are nearly corresponding to the test results: Mode I cracks basically propagated along the initial crack direction, while I/II mixed mode cracks initiate with a certain deflection angle and form wing cracks, which eventually propagate toward the major major principal stress direction. Obvious turning points exist propagation of mixed mode cracks. This study enables favorable observation of the fracture behavior of rock materials under impact loading. The experimental results are consistent with the stress and the strain data obtained from split hopkinson pressure bar (SHPB) test, indicating that the proposed method compensates for the deficiencies of SHPB system and exhibits satisfactory feasibility.