There exists some 1 000 m carbonaceous slate stratum under high geostress in the 9th inclined shaft work area of Guanjiao tunnel. The slate rock mass is weak, which would result in large deformation after excavation. Firstly, the main factors influencing the deformation of the tunnel and the plasticized zone of the surrounding rock are generally analyzed based on theoretical formula; Secondly, factors influencing the stability of the slate tunnel section, such as the parameters of the deformation and the strength of the rock mass, the geostress characteristics, the crosssection shapes and the support rigidity, are analyzed in detail by means of numerical simulation; Finally, a comprehensive evaluation index F is proposed based on the above analysis and the field experiments on the different crosssection shapes and different support rigidities. The index, which comprises geological condition, excavation crosssection shapes and support rigidity, is applied to evaluate the support of the tunnel in carbonaceous slate. The evaluation results coincide well with the deformation grade. The index can provide reference for the support system evaluation in the tunnel design stage.