Abstract: 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 resonably 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.