In order to investigate the characteristics of crack penetration in explosive bulk driven by burning products, and deepen the mechanism understanding on the process of high intensity reaction induced by burning products entering into cracks and forming convective burning under accidental ignition. The penetration of cracks in a HMX-based PBX (with a content of 95% for HMX) under thermal initiation at the preformed gap was photographed by the high-speed camera. Experimental results reveal that, due to the strong meso-heterogeneity of polymer bonded explosive (PBX), different samples of the same kind of explosive have different meso-structures, which results in the difference of crack propagation paths (which may propagate in a straight line or deflect)., Velocities of the crack penetration are nearly the same under same confinement and same pre-load stress. The average crack penetration velocity is about 146.7 m·s^-1 in the experimental condition of this research.