Lanthanide　(Ln3+)-enriched　upconversion　nanoparticles　(UCNPs)　with　high　dopant　concentrations　have　garnered　significant　attention　due　to　their　unique　optical　properties.　However,　their　practical　applications　are　hindered　by　the　deleterious　concentration　quenching　effect.　Herein,　through　kinetic　modeling　of　Er3+　excited-state　dynamics　employing　energy　diffusion　theories,　we　demonstrate　that　concentration　quenching　in　LiErF4　UCNPs　predominantly　originates　from　long-range　energy　migration　through　the　4I13/2　level　toward　surface　and　lattice　defects,　rather　than　the　conventionally　attributed　cross-relaxation　mechanism.　Such　migration-mediated　energy　dissipation　can　be　effectively　suppressed　by　the　synergistic　engineering　strategies　combining　surface　passivation,　spatial　confinement　via　a　sandwiched　LiYF4@LiErF4@LiYF4　core-shell-shell　architecture　to　restrict　Er3+　migration,　and　incorporation　of　Tm3+　as　energy　trapping　centers,　boosting　upconversion　quantum　yield　from　＜0.01%　to　2.29%　(980　nm@70　W　cm(-2)).　The　established　mechanistic　framework　and　material　design　principles　provide　critical　insights　for　engineering　heavily　doped　UCNPs,　particularly　advancing　their　application　potential　in　single-particle　spectroscopy　and　optoelectronic　nanodevices.