This　paper　presents　an　optimal　design　method　for　a　nonpacked　particle　damper　(NPPD)　based　on　a　simplified　mechanical　model　and　corresponding　damping　mechanism　analysis.　First,　an　equivalent　inertial　single-particle　mechanics　model　(EISM)　and　the　corresponding　equivalent　principle　were　proposed　to　consider　the　influence　of　particle　rolling　on　the　vibration　reduction.　The　experimental　verification　of　the　EISM　under　seismic　ground　motion　was　subsequently　carried　out.　Based　on　a　reasonable　verification　of　the　proposed　model,　an　analytical　solution　of　the　EISM　single-degree-of-freedom　structure　system　for　the　symmetric　two-impacts-per-cycle　motion　was　obtained.　The　influence　analyses　of　collision　distance　on　the　damping　effect　and　the　damping　mechanism　under　resonance　and　nonresonance　harmonic　excitation　were　performed.　The　theoretical　calculation　formula　for　the　optimal　collision　distance　for　energy　consumption　was　obtained　based　on　the　damping　mechanism　analysis.　Finally,　numerical　simulations　under　harmonic　excitation　and　seismic　ground　motion　were　carried　out　to　verify　the　rationality　and　generalization　of　the　proposed　optimal　collision　distance.　Accordingly,　an　optimal　design　method　for　the　NPPD　was　proposed.　Based　on　the　proposed　optimization　method,　the　vibration　reduction　effect　of　NPPD　under　far-field　records,　near-fault　pulse-like　records,　and　near-fault　nonpulse　records　are　discussed.　The　analysis　results　show　that　the　vibration　reduction　effect　of　the　NPPD　is　affected　by　the　response　frequency　of　the　uncontrolled　structure.　The　proposed　optimization　method　can　be　used　as　the　basis　for　the　optimization　of　NPPD　under　earthquakes.　©　2021　John　Wiley　&　Sons　Ltd.