It　is　not　uncommon　that　backfill　material　used　in　underground　mining　being　exposed　to　repetitive　dynamic　stresses　induced　by　blasting　operations　or　rockburst　events.　Understanding　the　strength　and　fracture　evolution　of　backfilled　stopes　is　critical　to　maintain　the　long-term　stope　stability　and　ensure　safe　mining　activities.　This　paper　aims　to　study　the　damage　evolution　of　the　backfill　material　and　its　host　rock　behaviour　under　three-dimensional　(3D)　dynamic　loading.　Using　a　true-triaxial　testing　machine,　multiple　samples　of　backfill　material　enclosed　by　country　rock　were　fabricated　and　tested　under　various　dynamic　loadings　with　different　true-triaxial　confining　stress　conditions.　In　addition,　the　nuclear　magnetic　resonance　(NMR)　measurement　was　conducted　on　the　samples　before　and　after　exerting　static　and　dynamic　loading　to　obtain　their　porosity　distribution　changes.　The　experiment　results　suggested　that　with　the　increase　of　the　dynamic　loading,　the　porosity　of　the　backfill　sample　goes　through　a　two-stage　process,　which　shows　a　slightly　linear　decrease　and　then　followed　by　an　exponential　increase.　The　research　findings　can　help　understand　the　damage　mechanism　and　fracture　development　of　backfilled　stopes　and　its　host　rock　in　deep　underground　mines,　which　are　constantly　subject　to　the　combination　of　3D　static　confining　stress　and　dynamic　loading.　(C)　2020　Published　by　Elsevier　B.V.　on　behalf　of　China　University　of　Mining　&　Technology.