Abstract: Experimental studies were preformed to investigate thecombustion and venting process of the methane/air mixture inside acylinder by the pressure transducers and the YA-16 high-speed shadowgraphsystems controlled by a synchronization unit. The pressure-time historiesand the sequences of high-speed shadowgraphs were obtained for the externalflow field, which can follow the development of the external explosions (the secondary explosions) in explosion venting. The experimentalresults show the typical characteristics of the external explosions. Thesecond pressure peak followed the first pressure peak formed by therupture shock wave. The shadowgraphs of the external flow field also showthe secondary explosion.