Abstract: This paper adopts the two methods: water absorption of irregular rock in both dry and saturated conditions and cemented coefficient of rock, proposed by Qu Yongxin, to study the swelling potential of mud rock. The mud rock is in some tunnel coal mine in Shanxi. The results show that the mud rock does not swell. On the other hand, the paper examines the mineral composition of the mud rock. It is found that the mud rock has much, non-uniform distributed quartz and iron, as well as kaolinite. The kaolinite is the main clay mineral. The mud rock shows brittle failure when it is compressed under uniaxial and triaxial tests with low confining pressure. Finally，the paper introduces several factors influencing the failure of rock. The factors include mineral composition, configuration and conformation, characteristic of joint or tiny joint surface and extension cranny in rock.