To　investigate　the　influence　of　shallow　buried　twin　flaws　on　the　failure　characteristics　of　underground　tunnels　surrounding　rock,　a　series　of　true　triaxial　compression　single　direction　unloading　tests　were　carried　out　using　sandstone　samples.　The　mechanical　properties,　crack　distribution　and　acoustic　emission　(AE)　response　features　of　three　kinds　of　fractured　samples　(two　open-type　flaws,　two　filled　flaws,　and　the　coexistence　condition)　were　studied.　According　to　the　stress-time-acoustic　emission　image　and　RA-AF　analysis　results,　the　evolution　of　cracks　and　deformation　characteristics　of　the　sandstone　are　investigated.　Compared　with　previous　similar　research,　the　main　results　and　conclusions　include:　(1)　Different　filling　conditions　of　the　preset　flaws　directly　impact　the　stress–strain　relationship,　AE　response　features,　mechanical　properties　and　failure　region　distribution.　(2)　Secondary　failures　caused　by　the　shallow　twin　flaws　develop　more　covertly,　and　no　typical　spalling　region　occurs　in　surrounding　rock.　(3)　According　to　the　results　of　the　RA-AF　analysis,　when　a　sample　contains　at　least　an　open-type　flaw,　tensile　failure　has　played　a　dominant　role　since　the　start　of　the　tunnel　excavation.　(4)　Small-scale　shallow　twin　flaws　can　lead　to　large-scale　destruction　of　the　free　face,　reduce　the　increase　in　sample　strength　from　uniaxial　compression　to　true　triaxial　(single　free　surface)　condition,　and　directly　affect　the　failure　patterns　of　rock　in　different　positions.　©　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Switzerland　AG　2025.