In order to explicitly solve the tooth surface model of forming face gear,and exactly solve the minimum inner radius of face gear with top-cut. The orthogonal face gear envelope coordinate system is established and the tooth surface equation of generating gear is given. The face gear working tooth surface equation,transition surface equation and top-cut surface equation are derived. According to the tooth surface equation,the simulation analysis of face gear tooth surface is carried out by using the MATLAB 7. 5 and CATIA V5. According to the simulation comparison of tooth surface and the analysis of top-cut boundary line,the top-cut minimum inner radius key point is got,the key point is the intersection of the face gear tip surface and top-cut boundary line. Through the explicit solving the key points,the minimum inner radius of the face gear without top-cut can be got. Comparing with the existing literature results of the implicitly solving inner radius,and the difference on numerical is small between the two. Finally,through the simulation cutting tooth way,it shows the accuracy of this method is superior to the method of literature.