Here, the dynamic propagation behavior of mode I and multi-mode I / II cracks in a tunnel under impact loads was investigated. A pre-crack was placed in a tunnel arch arc region and parallel to the symmetrical axis line of the tunnel. The ratio of the distance between the pre-crack and the symmetrical axis to the radius of the tunnel arch arc was in the range of 0~1. The dynamic impact tests of a sandstone tunnel model with a pre-crack were conducted with a middle-low speed drop weight impact test system. The crack initiation time and the propagation characteristic of the pre-crack were measured with strain gauges stuck at tips of the pre-crack. The software AUTODYN was used to simulate the tunnel’s crack propagation paths, and the numerical results agreed well with the test ones. The software ABAQUS was employed to calculate the crack’s dynamic stress intensity factor. The test-numerical method was used to determine the initiation toughness of the pre-crack in the tunnel model. The study results showed that in multi-mode I / II crack initiation process, there exist both mode I initiation toughness and mode II one, and their proportion is related to the distance between the crack and the symmetrical axis of the tunnel; the propagation behaviors of mode I and multi-mode I /II cracks are quite different, mode I crack along the pre-crack direction vertically upward expands, while multi-mode I / II crack initiates at a certain angle to the original crack to form an airfoil crack, and finally it expands along the maximum principal stress direction; in the propagation process of multi-mode I/II crack, there are obvious features of an inflection point.