A tunnel passes through completelyweatheredtointenselyweathered diabase, which is a kind of swelling rock mass. The difficulties in the construction of the tunnel located in the swelling diabase are studied by means of site monitoring and theoretical calculation. Conclusions drawn are as follows: 1) The weathered swelling diabase has such properties as containing a large amount of kaolinite, swelling when water is absorbed, shrinking when water is lost and being subject to spalling; 2) The tunneling may encounter such difficulties as uneven stress on the primary support, spalling of the primary shotcrete, serious deformation, failure and collapse; 3) Countermeasures to overcome abovementioned difficulties include controlling the deformation of the primary support, keeping good drainage conditions and preventing the diabase from contacting water. 