The relationship between crystal defects and hotspots formation of explosives under dynamic loading is a hot research topic in energetic materials. Understanding the mechanism of hotspots formation and its role in the ignition and sensitivity of high explosives are of great importance due to the needs of safety assessment of explosives and developing insensitive munitions. In this work， the ReaxFF reactive force field and molecular dynamics method were applied to investigate the dynamic responses of single crystal cyclotetramethylene tetranitramine （HMX） explosive with cylindrical voids under shock loading. Moreover， the effect of void size and double voids were studied. It is found that the shock induced collapse of voids includes three stages， namely， the plastic deformation on the upstream of the void， the movement of upstream atoms towards the centerline and the downstream of the void forming flowing atoms， and the collision of flowing atoms on the downstream. The main mechanism of hotspots formation is the collision of flowing atoms on the downstream that transforms kinetic energy into thermal energy leading to rapid temperature rise. The high temperature of hotspots initiates local chemical reactions， and the breaking of N─NO₂ bond in HMX molecule with NO₂ formation is the principal initial reaction mechanism. The void collapse process and hotspots formation mechanism of cylindrical void is similar to spherical void， while the convergence effect is weaker and the velocity of formed flowing atoms is lower for cylindrical void， resulting in significantly low hotspot temperature and weak chemical reactions. Besides， the collapse of cylindrical void forms shear bands around， which was not observed for spherical void. With the increase of void size， the velocity of flowing atoms goes up， the shear bands are wider， and the hotspot temperature is higher and hotspot area is larger， leading to more violent chemical reactions. For the sample with two voids that are aligned along the shock direction with a distance of void radius， the collapse of voids is similar to single void. The shock pressure reduces when the shock wave propagates through the upstream void due to the reflected rarefaction wave. Therefore， the velocity of the flowing atoms formed during the collapse of the downstream void is smaller， and the temperature of the second hotspot is lower. The current findings are beneficial to comprehend the effects of crystal defects on hotspots formation and subsequent ignition of explosives and can provide physical mechanism and laws cognition to construct macro-theoretical models.