As　an　innovative　and　efficient　rock　breaking　method,　free　face-assisted　rock　breaking　is　usually　applied　to　tunnel　excavation　with　TBM　multi-stage　cutterhead.　To　reveal　the　free　face-assisted　breaking　mechanism　of　soft　rock,　the　finite-discrete　element　method　(FDEM)　is　adopted　to　study　the　mudstone　fragmentation　characteristics,　cutter　force,　acoustic　emission　(AE)　and　rock　breaking　efficiency.　Then,　the　numerical　tests　of　mudstone　samples　with　different　free　face　conditions　under　TBM　cutter　indentation　are　conducted.　The　variations　of　fragmentation　pattern,　rock　breaking　angle　and　maximum　cutter　force　with　free　face　spacing　and　height　are　analyzed.　Finally,　a　fragment　extraction　algorithm　is　used　to　identify　rock　fragment　distribution.　The　fragment　area,　cutter　work　and　specific　energy　(SE)　are　calculated.　Results　show　that　free　faceassisted　fragmentation　has　a　higher　rock　breaking　efficiency　with　more　complete　and　largerarea　fragments　compared　with　traditional　breaking　method.　There　are　three　rock　fragmentation　patterns　controlled　by　several　critical　values　of　free　face　spacing　and　height.　Besides,　a　spacing　of　75　mm　and　a　height　of　125　mm　can　bring　the　largest　fragment　area　and　the　highest　rock　breaking　efficiency.　The　results　can　provide　theoretical　guidance　for　the　design　of　TBM　multi-stage　cutterhead　for　efficient　tunneling　in　soft　rock　strata.