Abstract: In order to obtain the ignition behavior of PBX molding powder under gap extrusion loading, an experimental device for gap extrusion of molding powder was designed based on projectile impact. In order to ensure that there is no other flow space except the designed gap, the surface of the sample was covered with cushion and coated with grease for sealing, and the movement and reaction of molding powder squeezing into the gap were recorded by high-speed photography. By changing the ratio of gap area to sample cross-sectional area, the influence of compaction on ignition was studied. The results show that in the absence of grease seal, PBX molding powder undergoes particle crushing and compaction, and then the compacted molding powder is extruded from the clearance near the cushion, and ignition occurs in the extrusion process. The ignition position is at the interface between explosive and cushion. In the case of grease seal, PBX molding powder does not ignite for a period of time after compaction. When the indenter moves halfway, a “wedge-shaped” slip zone is formed, and a slip-dead zone interface could be seen in high-speed camera photos. Then the deformation mode evolves from “single wedge” slip zone to “double wedge” slip zone, and the shear effect of slip-dead zone interface does not cause ignition. At the later stage of loading, the indenter travels close to the gap surface, and the “wedge-shaped” slip zone disappears. Before and after the collision between the indenter and the gap, the explosive ignites once, respectively. The first ignition occurs at the entrance of the gap, and the second ignition occurs at the corner of the indenter. Compaction effect has an important influence on ignition behavior. After compaction, the threshold value of ignition speed is obviously reduced, and the impact speed causing ignition is only 4.5 m/s.