Abstract The destabilization of coal body containing gas that is caused by the quick propagation of the cracks in coal body containing gas and the high pressure gas, which result into coal and gas outburst. Thus, according to the theory of brittleness rupture mechanics the propagation speed calculation formula of the cracks in coal body containing gas is derived under constant stress and the mechanism of the cracks propagation is analyzed. The propagation speed of the cracks in coal body containing gas is calculated by the experimental work. The research results show that gas pressure make the cracks propagation easier, which is consistent with the analysis result of the propagation speed calculation of the cracks. As well as the propagation speed of the cracks increase with the augment of gas pressure.