To explore the cause and mechanism of the disaster caused by abnormal gas emission in coal seams with low-pressure gas, the literature in this field was analyzed and summarized, and the existence of such disaster was verified by case study. Based on the fluid-solid coupling experimental system, the physical test of coal sample bearing pressure with low pressure was carried out according to the constraints and loading conditions of the engineering, and the overall stress-acoustic emission-flow experimental data and the external damage of the sample were obtained. Using UDEC software, the discrete element numerical experiments of gas-solid coupling were carried out according to the boundaries and loading conditions of the engineering, and experimental data such as stress-strain-displacement rate and internal cracking of the sample were gained. Results show that the three methods had a high consistency. With the gas pressure of 0.4 MPa, gas outburst occurred under stress-coal-gas coupling, and it generally appeared as the amount of gas emission exceeding the limit in coal mine. The unidirectional pressure load was applied under the single-degree-of-freedom boundary condition. At the early stage of the elastic deformation of the pressure-bearing coal rock, the gas flowed along initial cracks and pores at a normal steady speed. At the later stage of the elastic deformation, the initial cracks and pores were compacted and gas flowed at a slow steady speed. During the yield phase, the new fissures triggered by coal dilatancy were formed and gas flowed rapidly and unsteadily. When the coal mass bore approximately achieved ultimate load or shortly reached the peak, gas outburst disaster happened. The phenomenon of continuous decrease of gas concentration faced in the mining and excavating work under normal ventilation is a warning sign that coal seams have abnormal gas emission, and to which great importance should be attached.