A　non-packed　particle　damper　(NPPD)　is　an　advantageous　passive　damping　device　with　a　wide　frequency　band　and　good　robustness.　However,　its　damping　mechanism　remains　unclear,　and　its　application　for　controlling　vibrations　in　civil　engineering　structures　is　being　explored.　By　analysing　the　particle　motion　state,　this　paper　proposes　a　double　inertia　mass　equivalent　model　(DIMEM)　to　reflect　the　interaction　between　particles.　The　effect　of　inertia　was　introduced　in　the　DIMEM　to　consider　the　rolling　effect　of　particles　on　the　damping　performance　of　the　NPPD.　An　electromagnetic　vibration　table　test　of　a　single-storey　frame　structure　with　an　NPPD　was　designed　and　performed.　The　parameter　determination　method　and　applicability　of　the　DIMEM　were　also　evaluated　based　on　experimental　and　theoretical　analyses.　Furthermore,　the　effects　of　the　parameters　on　the　damping　performance　of　the　NPPD　were　systematically　analysed.　The　results　confirm　that　the　equivalent　model　can　reflect　the　damping　performance　and　mechanism　of　the　NPPD.　The　particle　group　inertia　is　an　inherent　property　of　the　NPPD.　With　an　increase　in　the　ratio　of　inertance　to　particle　mass,　the　optimal　vibration　damping　performance　deteriorates　under　harmonic　excitation,　and　the　corresponding　equivalent　connection　stiffness　gradually　increases.　The　equivalent　connection　stiffness,　additional　mass　ratio,　recovery　coefficient,　and　acceleration　normalised　collision　distance　significantly　affect　the　damping　performance　and　mechanism.　However,　the　effects　of　the　equivalent　connection　damping　coefficient　and　rolling　friction　coefficient　are　negligible.　The　proposed　DIMEM　and　the　corresponding　parameter　analysis　can　provide　an　important　reference　for　the　application　of　NPPDs　in　civil　engineering.　©　2020