Abstract: The engineering property of soils is in substance determined by their microstructure with embodies unit property of soils. Scanning electron microscope equipment and mineral X-ray diffraction equipment are used to study the microstructure of low liquid limit silt. The engineering property of the low liquid limit silt is evaluated from the mineral ingredient and the structure by virtue of microstructure. The low liquid limit silt presents different structural and coupling modes under different compaction degrees. Different arrangements of particles from inside to outside are characterized by the void ratio. So enhancing the compaction degree alters the coupling modes of silt and the arrangement sequence of silt particles so as to increase the engineering property of the low liquid limit silt.