During　tunnel　blasting　excavation,　empty　holes　act　as　auxiliary　free　surfaces　and　gas　compensation　spaces,　and　these　features　significantly　influence　blasting　performance.　To　clarify　how　empty-hole　diameter　(D)　and　the　spacing　between　empty　and　charge　holes　(l)　affect　outcomes,　a　three-dimensional　numerical　model　was　built.　This　model　used　the　SPH–FEM　(smoothed　particle　hydrodynamics　coupled　with　finite　element　method)　approach.The　effects　of　D　and　l　on　the　blasting　response　are　examined　by　comparing　rock　damage　evolution,　particle　ejection　velocity,　debris　migration,　and　the　projected　area　of　the　cut　cavity　for　various　parameter　combinations　(D　=　105　~　210 mm,　l　=　400　~　600 mm).　The　results　identify　key　mechanisms　that　control　blasting　performance.　Increasing　D　and　l　appropriately　expands　the　cut　cavity　and　reduces　particle　ejection　velocity.　When　D　is　1.5　times　the　charge-hole　diameter　and　l　is　three　times　the　empty-hole　diameter,　the　fragmentation　pattern　is　optimal,　the　cut　cavity　is　well-formed,　and　explosive　use　is　significantly　improved.　This　parameter　combination　offers　quantitative　guidance　for　cut　blasting　design　and　has　important　engineering　implications.　©　The　Author(s),　under　exclusive　licence　to　Springer　Nature　Switzerland　AG　2025.