Plasmonic　structure　color　has　significant　potential　for　visual　biochemical　sensing　by　simple　instrumentation　or　even　naked　eye　detection.　Herein,　we　present　a　visual　and　real-time　sensing　strategy　for　refraction　index　sensing　and　detection　of　the　biotin-avidin　system　based　on　three-dimensional　cavity-coupled　metamaterials.　These　metamaterials　composed　of　a　top　array　of　gold　disks,　aluminium　pillars　and　a　bottom　reflection　film　of　aluminium　have　structures　similar　to　the　metal-insulator-metal　structure.　The　insulating　layer　comprises　air-gap　cavities　that　are　easily　filled　with　gaseous　or　liquid　dielectrics.　Therefore,　analytes　can　permeate　into　the　nano-scale　cavities　and　produce　strong　light-matter　interactions.　The　sensor　shows　that　any　tiny　change　in　the　refraction　index　will　induce　a　significant　color　variation　and　the　sensitivity　reaches　683.5　nm　per　refraction　index　unit　with　a　figure　of　merit　of　3.5.　The　color　of　the　metamaterials　changes　from　rose-red　to　violet　and　then　loden　after　a　monomolecular　layer　of　thiolated　biotin　and　streptavidin　bind　to　the　surface　of　the　nanostructure　successively.　This　sensing　strategy　offers　new　opportunities　for　the　convenient　detection　of　proteins,　nucleic　acids,　and　lipids.　©　2020　The　Royal　Society　of　Chemistry.