Abstract: In order to study the failure process of coal deformation and the expansion law of plastic zone under gas pressure, reduce the risk of coal and gas outburst and ensure the safety production of coal mine. Based on digital image reconstruction technology and using FLAC^3D finite difference software, the instability failure process of coal sample under unconfined compression was numerically simulated, and the influence of gas pressure on peak strength and failure pattern of coal was analyzed. The results show that the no gas-containing coal sample is mainly shear failure, and there is a main shear failure surface running through the whole model; the instability failure of coal is the result of the interaction of external load and internal gas pressure, there are two failure forms in the gas-containing coal sample, that is, tensile failure is added on the shear failure; the peak strength and peak strain of the gas-containing coal sample decrease linearly with the increase of gas pressure. Combining digital image reconstruction technology and finite element program, a numerical model reflecting the real structure of coal can be established, which provides effective means for studying the influence of coal and rock heterogeneity on the mechanical properties of coal.