Production　conditions　and　formation　mechanism　of　shear　band　were　analyzed　based　on　modified　cam　model　in　finite　element　program　ABAQUS.　The　results　show　that　strain　softening　is　not　the　necessary　condition　of　shear　band　formation　in　soil.　Non-uniform　force　induced　by　end　restraint　and　initial　defect　can　cause　non-uniform　deformation,　which　is　the　necessary　condition　of　shear　band　formation.　The　physical　mechanism　of　shear　band　formation　is　presented　that　non-uniform　force　or　defect　element　causes　the　non-uniform　deformation　in　soil.　If　the　critical　bearing　capacity　of　elements　on　some　shear　surface　is　reached,　strain　energy　of　other　elements　will　rapidly　release　while　deformation　slightly　increase.　The　impact　action　caused　by　strain　energy　releasing　leads　to　rapid　deformation　increase　and　load　reduction　of　elements　on　this　section,　and　then　continuous　visible　shear　band　is　formed.