Eco-friendly building materials with perfect thermal insulation & sound absorption property have become intriguing and eye-catching in recent years. In this work, the ultra low-density binderless sandwiching materials were firstly fabricated with ultra low-density of 60-80 kg/m3 by self-designed rapid steam injection technology. The main experimental factor of density, holding time, transmission time, steam injection pressure and fiber’s dimension was respectively investigated to their effects on formation of the new building materials. IR, Py GC-MS and AFM analysis were performed to study the mechanism of binderless sandwiching materials under steam injection process. The bending strength, thermal insulation & sound absorption property of the new materials were also studied. This new building material with no resin use and no formaldehyde release is expected to be reserved as the sandwich for designing thermal insulation & noise reduction building materials.