Controllable　strong　interactions　between　a　nanocavity　and　a　single　emitter　is　important　to　manipulating　optical　emission　in　a　nanophotonic　system　but　challenging　to　achieve.　Herein　a　three-dimensional　DNA　origami,　named　as　DNA　rack　(DR)　is　proposed　and　demonstrated　to　deterministically　and　precisely　assemble　single　emitters　within　ultra-small　plasmonic　nanocavities　formed　by　closely　coupled　gold　nanorods　(AuNRs).　Uniquely,　the　DR　is　in　a　saddle　shape,　with　two　tubular　grooves　that　geometrically　allow　a　snug　fit　and　linearly　align　two　AuNRs　with　a　bending　angle　＜　10　degrees.　It　also　includes　a　spacer　at　the　saddle　point　to　maintain　the　gap　between　AuNRs　as　small　as　2-3　nm,　forming　a　nanocavity　estimated　to　be　20　nm(3)　and　an　experimentally　measured　Q　factor　of　7.3.　A　DNA　docking　strand　is　designed　at　the　spacer　to　position　a　single　fluorescent　emitter　at　nanometer　accuracy　within　the　cavity.　Using　Cy5　as　a　model　emitter,　a　similar　to　30-fold　fluorescence　enhancement　and　a　significantly　reduced　emission　lifetime　(from　1.6　ns　to　670　ps)　were　experimentally　verified,　confirming　significant　emitter-cavity　interactions.　This　DR-templated　assembly　method　is　capable　of　fitting　AuNRs　of　variable　length-to-width　aspect　ratios　to　form　anisotropic　nanocavities　and　deterministically　incorporate　different　single　emitters,　thus　enabling　flexible　design　of　both　cavity　resonance　and　emission　wavelengths　to　tailor　light-matter　interactions　at　nanometer　scale.