Numerous　studies　have　shown　that　the　acoustic　metamaterials　have　high　the　sound　insulation　performance　at　the　anti-resonant　frequencies.　On　the　basis　of　the　sub-wavelength　locally　resonant　principle　of　acoustic　metamaterials,　we　proposed　a　novel　ultrathin　lightweight　membrane-type　acoustic　metamaterial　(MAM)　structure　unit　with　polymorphic　anti-resonance　modes,　which　would　achieve　an　excellent　ability　to　insulate　the　broadband　noise　between　50　Hz　and　736　Hz,　besides,　the　sound　transmission　loss　(STL)　amplitude　has　been　promoted　to　48　dB.　STL　was　numerically　assessed　and　experimentally　tested　using　a　small　sample　(one　cell)　inside　an　impedance　tube.　Then,　STL,　modal　displacement　distributions　and　effective　parameters　of　the　MAM　unit　were　researched　to　reveal　the　localized　surface　anti-resonance　characteristics　and　the　formation　mechanism　of　STL　peak　and　bandwidth.　As　to　the　designed　sample,　the　design　approach　with　special　mass　disks　distributed　on　the　surface　of　the　membrane　generated　the　polymorphic　bending　vibration　modes　among　the　mass　disks　and　the　membrane.　The　results　indicated　that　the　whole　composite　structure　unit　becomes　the　strongly　local　resonance　modes　-　retrorse　lumped　coupling　resonance　and　polymorphic　anti-resonance　-　with　the　incident　sound　waves,　followed　by　a　high　sound　attenuation,　the　excellent　agreement　between　theory　and　experiment　is　obtained.　©　INTER-NOISE　2018　-　47th　International　Congress　and　Exposition　on　Noise　Control　Engineering:　Impact　of　Noise　Control　Engineering.　All　rights　reserved.