A high strain rate dynamic mechanical compression test was carried out on an as-cast ZM6 magnesium alloy by using a split Hopkinson pole, and the mechanical properties and microstructure of the resulting ZM6 magnesium alloy were studied and analysed. The results show that the strain rate and the maximum strain of the specimen increase with increasing impact load before disintegration. According to the true stress-strain curve, the as-cast ZM6 magnesium alloy exhibits obvious positive strain rate strengthening. With increasing strain rate, the work hardening effect is significant, and the maximum impact strength reaches 328.1 MPa. According to the microstructural analysis, the fracture mode of the as-cast ZM6 magnesium alloy at a high strain rate is brittle fracture.