A　theoretical　model　was　set　up　for　analyzing　the　mechanical　characteristics　of　the　surrounding　rock　in　karst-tunnel　system,　and　Schwarz＇s　alternative　method　was　employed　to　derive　the　analytic　solutions　for　the　stress　and　deformation　of　the　surrounding　rocks　upon　excavating　deep　buried　tunnel　under　the　existing　karst　conditions.　Taking　Qiyueshan　railway　tunnel　for　example,　the　analytic　results　of　the　mechanical　characteristics　of　surrounding　rock　were　compared　with　finite　element　numerical　results,　which　validated　the　correctness　of　the　process　for　deriving　the　analytic　solutions　of　the　model.　Analysis　results　show　that　karst　cave　results　in　bias　pressure　phenomenon　to　the　surrounding　rock　during　tunnel　excavation　process.　There　is　obvious　vertical　stress　concentration　at　the　surrounding　rock　near　karst　cave,　and　there　is　little　difference　between　the　horizontal　stresses　of　two　sides.　Greater　horizontal　displacement　occurs　to　the　surrounding　rock　near　karst　cave.　The　ratio　of　the　maximum　values　between　two　sides　of　tunnel　is　about　1.7.　With　the　increasing　of　the　clear　distance　between　tunnel　and　karst　cave,　the　bias　pressure　of　surrounding　rock　weakens　gradually.　The　effect　of　karst　cave　on　the　surrounding　rock　in　deep　buried　tunnel　can　be　neglected　when　the　clear　distance　between　tunnel　and　karst　cave　is　greater　than　1.5　times　of　cave　diameter.