Abstract: The instability behavior and static liquefaction of loose sand are investigated by conducting a series of discrete element method (DEM) simulations in biaxial shear tests. The essential factors affecting the initiation of instability behavior are examined from both macroscopic and microscopic perspectives. The numerical simulation results reveal that the instability behavior of loose sand relies on the initial void ratio and fabric anisotropy as well as confining pressure. It is found that the initiation of instability is generally accompanied by the rotation of principal direction of fabric anisotropy if the loading direction does not coincide with the principal direction of fabric anisotropy which is characterized by normal contact unit. The larger the deviation between these two directions is, the easier the initiation of instability behavior becomes and the more intense particle rearrangements and microstructure reorganizations are expected to be as well. At the instability state, most deviatoric shear loads are carried by normal forces at contacts, and tangential forces merely play a secondary role.