In　order　to　solve　the　problems　of　tunnel　instability　caused　by　poor　geological　weak　surrounding　rock,　the　uncertainty　of　grouting　parameters　of　weak　surrounding　rock,　and　the　influence　of　grouting　thickness　of　surrounding　rock　of　ice　water　accumulation　body　of　Milin　tunnel　on　the　safety　of　support　force,　the　tunnel　surrounding　rock　model　with　different　grouting　thickness　is　established　based　on　the　three-dimensional　finite　element　FLAC3D　simulation　software.　The　surrounding　rock　characteristic　curve　after　grouting　is　obtained,　and　the　allowable　support　force　corresponding　to　different　grouting　thickness　is　determined　through　the　allowable　deformation.　On　this　basis,　the　influence　of　different　grouting　thickness　of　surrounding　rock　on　the　primary　support　structure　is　further　discussed,　and　the　safety　factor　of　primary　support　at　each　position　of　the　section　is　calculated.　The　results　show　that:　(1)　The　greater　the　thickness　of　the　grouting　circle,　the　smaller　the　allowable　support　force　based　on　the　allowable　deformation;　and　the　worse　the　surrounding　rock　conditions,　the　greater　the　allowable　support　force　corresponding　to　the　same　grouting　thickness.　It　is　suggested　that　the　reasonable　thickness　of　the　grouting　circle　for　the　Ⅳ1,　Ⅴ1　and　Ⅵ　surrounding　rocks　of　the　ice　water　accumulation　section　of　the　Milin　tunnel　should　be　3.5　m,　3.5　m,　and　5.0　m,　respectively;　(2)　With　the　increase　of　the　thickness　of　grouting　circle,　the　safety　factor　of　primary　support　corresponding　to　each　position　of　surrounding　rock　at　all　levels　increases.　(3)The　safety　factor　of　primary　support　is　generally　large　at　the　arch　spandrel,　crown,　and　bottom,　and　minimum　at　the　arch　waist　and　foot.　©　2022,　Editorial　Office　of　Tunnel　Construction.　All　rights　reserved.