This　research　proposes　a　novel　type　of　variable　stiffness　tuned　particle　damper　(TPD)　for　reducing　vibrations　in　boring　bars.　The　TPD　integrates　the　developments　of　particle　damping　and　dynamical　vibration　absorber,　whose　frequency　tuning　principle　is　established　through　an　equivalent　theoretical　model.　Based　on　the　multiphase　flow　theory　of　gas-solid,　it　is　effective　to　obtain　the　equivalent　damping　and　stiffness　of　the　particle　damping.　The　dynamic　equations　of　the　coupled　system,　consisting　of　a　boring　bar　with　the　TPD,　are　built　by　Hamilton＇s　principle.　The　vibration　suppression　of　the　TPD　is　assessed　by　calculating　the　amplitude　responses　of　the　boring　bar　both　with　and　without　the　TPD　by　the　Newmark-beta　algorithm.　Moreover,　an　improvement　is　proposed　to　the　existing　gas-solid　flow　theory,　and　a　comparative　analysis　of　introducing　the　stiffness　term　on　the　damping　effect　is　presented.　The　parameters　of　the　TPD　are　optimized　by　the　genetic　algorithm,　and　the　results　indicate　that　the　optimized　TPD　effectively　reduces　the　peak　response　of　the　boring　bar　system.