In　the　present　study,　the　effects　of　Nb　addition　on　Charpy　impact　properties　of　TiVTa　refractory　high-entropy　alloy　with　high　strength-ductility　trade-off　were　systematically　studied　by　using　the　instrumented　Charpy　impact　testing　machine.　The　experimental　results　showed　that　the　impact　toughness　was　remarkably　improved　by　Nb　addition　in　TiVTa　to　form　TiVTaNb　alloy.　The　crack　initiation　energy　and　propagation　energy　of　TiVTaNb　were　67.3％　and　24.9％　higher　than　that　of　TiVTa,　indicating　that　Nb　addition　simultaneously　reinforced　the　resistance　to　crack　initiation　and　propagation.　The　impact　fracture　of　TiVTaNb　exhibited　larger　bending　degree　of　shear　lips,　deeper　dimples　and　more　secondary　cracks　which　effectively　dissipated　more　impact　energy.　The　deformation　mechanism　of　TiVTa　alloy　was　dominated　by　dislocation　activities.　While　in　TiVTaNb,　the　deformation　mechanism　was　synergized　by　dislocation　activities　and　deformation　twinning,　which　were　the　main　contributors　for　the　improved　impact　properties　and　the　stronger　crack　resistance　of　TiVTaNb　alloy　under　impact　loading.