Since karst landform distributes widely in south China, highway and railway construction usually adopt the method of embedding long and large tunnel in great depth so that they could pass these karst areas safely. Water outburst and mud outburst accidents occurring on the stable karst area are major hazards in highway and railway construction and operation. The grave mud outburst accident occurring at the entrance of Yuanliangshan Tunnel has the following features: (1) the dissoluble rocks distribute widely, possessing a sound catchment capacity. (2) Water storage structure exists above the elevation of tunnel body; (3) feature of geologic structure and attitude of stratum in that area benefit the development of karst; (4) underground karst pipe is smooth and unobstructed. And there are certain underground rivers. (5) Besides, the thickness of water-proof layer has less power than the hydraulic pressure of ground water. (6) Karst mud outburst hazard features suddenness and concealment. Taken the example of Yuanliangshan Tunnel, the paper conducts preliminary analysis and careful calculation of the speed of karst mud outburst and its impact force, providing some research ground for disaster assessment and prevention.