Abstract: The occurrence of coal and gas outburst (abbreviated as “outburst”) is a dual unloading mode of simultaneous unloading of gas pressure and surrounding pressure. In order to determine the main controlling effects of stress and gas pressure on the failure of outburst coal and clarify its mechanism in the process of outburst, based on the triaxial seepage test, the mechanical state, permeability characteristics, and tendency of failure for coal specimens under various unloading modes and different unloading ratios were analyzed. The mechanism of the effective stress and Klinkenberg effect under various unloading modes was discussed. The results show that the axial strain of coal continuously increases under the unloading of gas pressure, confining pressure, and dual unloading modes. When only the gas pressure is unloaded, the radial strain decreases linearly, resulting in a 36.88% increase in the final permeability rate of the coal. The Mohr stress circle radius remains unchanged, and the stress circle gradually moves away from the strength envelope line, and the specimen gradually tends to stabilize. When only the surrounding pressure is unloaded, the radial strain increases linearly, resulting in a 990.91% increase in the final permeability rate of the coal, and at the same time, the Mohr stress circle radius gradually increases and the Mohr circle approaches the strength envelope line, leading to failure of the specimen. Under the dual unloading mode, in the early stage of unloading, the strain ratio of the body under the dual unloading mode is the largest, and the final permeability rate of the coal is 149.24%. The Mohr stress circle radius gradually increases, the Mohr circle moves away from the strength envelope line, and the specimen gradually tends to stabilize. However, with the increase in the unloading ratio of the dual unloading mode, the rates of axial strain, radial strain, body strain of the coal increase, the dominant role of effective stress in the outburst process is enhanced, leading to a higher rate of permeability increase, the corresponding increase in the radius of the stress circle gradually increases, and the stress circle will gradually move from being far away from the envelope line to being close to the envelope line. The outburst is characterized by multiple intermittent processes, under the dual control of stress and gas pressure. When typical coal-gas two-phase flow phenomena occur, the coal exhibits a dual unloading process, and stress plays a dominant role in the development of the outburst. During the brief stoppage stage of the outburst, the coal exhibits a merely gas pressure unloading process, at which point the gas pressure plays a dominant role in the re-initiation of the outburst.