Abstract: Vented explosion tests in a 0.00814m3 steel cylindrical vessel connected to a duct, filled with stoichiometric ratio of CH4-Air, have been performed with central ignition in the end plane and the failure pressure 230±15KPa. Moreover it was simulated by using colocated grids SIMPLE schemes, based on the k- turbulent model and EBU combustion model. The pressure-time profiles at the four measurement points along the outside axis and the sequential shadowgraphs of the external flowfield both in test and simulation were clear to show the generation and development of the rupture shock and secondary explosion wave. The calculated results were in good agreement with the measured results in trend. At the early stage of venting, there existed rupture shock wave, low pressure area and suspended shock (high pressure area) about the axis in the external flowfield, which was formed for the convergence of the reflected rarefaction wave from the boundary layer. At the same time, the high pressure area was also the high density pre-heating area for the combustible gas compressed. When the jet flame came out and penetrated into the high pressure area, the secondary explosion was formed for the violent combustion with the interaction of the turbulence, barocline effect etc. In this paper,based on the test and numerical results, the main mechanisms of the occurrence of secondary explosions was elucidate systematically.