In this paper,we study the approximation of involute profile by moor-shaped ring section. By using the minimax method, we find parameter of the best approximation moon-shaped ring section of invelute piofile. The error of approximation is 35% - 70% that of [1] and [2]. Thus the approximation precision of dressing a wheel for involute gear forming-grinding by approximating method has been greatly improved. In order to simplify calculation, the error function is defined according to the implicit equation of curve and its rationalization is proved.