Because the ammunition may be affected by the impact of multiple fragments, the penetration effect of single and multiple fragments impacting the shell charge. The finite element model is simulated by AutoDyn-3D. The process on different shapes single fragment and double fragments impacted the shelled explosive and shock detonation were simulated, calculated the threshold speed of impacting shelled explosive, and analyzed the impact of different gap on shock detonation. The threshold speed and the change rule of the detonating with shelled explosive under different conditions were obtained. The results show that the diameter, the threshold speed and the distance between the two fragments have a certain effect on the impact initiation of the shelled explosive. As the spacing increases, the pressure peak decreases and the attenuation becomes faster.