Cutter　spacing　is　one　of　the　key　factors　affecting　the　rock　breaking　efficiency　of　Tunnel　Boring　Machine　(TBM).　In　the　excavation　of　deep　soft　rock　tunnels,　the　water　content　can　change　the　fragmentation　behavior　by　TBM　cutters,　and　further　influence　rock　breaking　efficiency.　This　study　takes　mudstone　as　an　example　to　study　the　influence　mechanism　of　water　content　on　the　optimum　cutter　spacing　and　rock　breaking　efficiency　when　TBM　disc　cutters　penetrate　soft　rock.　Firstly,　the　FDEM　water　migration-fracture　coupled　model　considers　the　water　weakening　effect　based　on　existing　experimental　fitting　results.　Secondly,　several　numerical　experiments　are　conducted　to　calibrate　the　microscopic　parameters　input　in　FDEM.　Finally,　a　series　of　numerical　experiments　of　TBM　double　disc　cutters　penetrating　in　water-soaked　mudstones　are　carried　out　using　the　coupling　model.　The　crushing　characteristics,　cutter　force,　acoustic　emission　(AE),　and　crack　pattern　with　different　water　content　and　cutter　spacing　are　discussed　in　detail.　Moreover,　a　novel　recognition　algorithm　is　proposed　to　calculate　the　rock　chip　distribution　and　area.　The　crack　number,　work　done　by　the　cutters,　and　specific　energy　(SE)　are　quantitatively　analyzed.　The　results　indicate　that　the　water　content　of　2%–3%　and　the　cutter　spacing　of　60–80　mm　can　lead　to　the　lowest　SE　and　highest　rock　breaking　efficiency.　Overall,　the　optimum　cutter　spacing　decreases　with　the　increase　of　water　content.　The　modeling　results　can　supply　guidance　for　the　optimization　of　cutter　spacing　during　TBM　tunneling　in　water-rich　soft　rock　ground.　©　2023　Elsevier　Ltd