Abstract: N2 or CO2 is commonly injected into coal seam to enhance CBM recovery or realize CO2 storage. The injected gas will make crack growth occur in coal seam, which can induce instability. Taking the No. 9 coal seam in Ningwu basin as the research object, firstly, the isothermal adsorption experiment was carried out to study the adsorption capacity of coal rock for N2 and CO2 quantitatively and tri-axial compression mechanics experiment was carried out to study the influence of N2 and CO2 adsorption on the mechanical strength of coal rock. The mechanism of crack propagation was analyzed and the crack propagation speed equation of coal seam containing gas was deduced according to the theory of fracture mechanics and, according to the equation, the crack propagation speed was calculated when different gases were injected. Studies show that the adsorption of coal rock for CO2 is six times larger than CH4 under the same equilibrium pressure, the mechanical strength of coal rock saturated by CO2 is significantly lower than that of N2, The crack propagation speed of coal with CO2 injection is much bigger than that of N2, and the greater the gas injection pressure is, the higher the propagation speed will be. This theoretical result can not only provide a guidance for optimizing the proportion of gas injection and gas injection pressure, but also is of great significance to prevent the instability of coal bed and ensure gas injection smoothly.