Slabbing　is　a　failure　phenomenon　that　the　surrounding　rock　mass　is　separated　into　parallel　slabs　because　of　splitting　effect　in　underground　excavation.　It　always　occurs　in　intact　or　relatively　intact,　hard,　brittle　and　deep　buried　surrounding　rock　mass.　The　slabs　are　always　arc-shaped　in　circular　tunnels.　When　an　arc-shaped　slab　experiences　a　bending　deformation,　which　directs　to　the　external　of　a　tunnel,　decreases　the　radius　of　the　slab　and　increases　its　curvature,　this　kind　of　bending　deformation　can　be　named　as　concave　deformation.　In　the　case　of　the　drainage　tunnel　excavated　by　TBM　in　Jinping　II　Hydropower　Station,　the　existence　of　arc-shaped　slabs　owing　to　splitting　in　the　circular　tunnel　is　definitely　pointed　out　according　to　the　in-situ　characteristics　of　the　surrounding　rock　mass,　and　the　possibility　of　the　concave　deformation　of　the　arc-shaped　slab　is　put　forward　as　well.　Then　the　possibility　of　the　concave　deformation　is　demonstrated　by　means　of　mechanical　analyses　and　numerical　tests　respectively.　The　demonstration　reveals　that　the　occurrence　of　the　concave　deformation　is　dominated　by　the　stress　difference　between　vertical　and　horizontal　geostresses　which　press　the　circular　tunnel　into　an　approximatively　elliptical　one.　The　demonstration　of　the　concave　deformation　for　the　arc-shaped　slab　will　play　an　important　role　in　intensive　studies　on　mechanism　of　dynamic　failure　of　deep　and　intact　rock　mass　in　circular　tunnels.