Tooth　stiffness　of　involute　internal　spur　is　treated　as　a　constant　in　ISO,　but　changes　with　time　in　fact.　In　order　to　deduce　a　simplified　approximate　formula　to　calculate　the　tooth　stiffness　of　the　internal　spur　gear,　the　paper　sets　up　an　accurate　3D　solid　model　of　internal　gear　in　the　involute　profile,　and　it　reasonably　selects　and　proposes　the　following　four　main　gear　parameters　which　influence　the　tooth　stiffness　of　the　internal　spur　gear:　position　of　load　action,　number　of　teeth,　addendum　modification　and　tool　edge　radius.　The　paper　compares　the　value　of　the　tooth　position　of　load　action　with　the　value　of　the　symmetric　center　point　to　identify　which　one　is　the　value　of　deformed　deflection　of　the　gear　tooth　in　FEM;　Then,　the　deformed　deflection　of　the　gear　tooth　is　practically　calculated　by　FEM;　At　last,　a　simplified　approximate　formula　to　calculate　the　tooth　stiffness　of　the　internal　spur　gear　is　deduced　by　the　regressive　analysis　of　the　data.