This paper introduces the machining principles and characteristics of mold grinding cylinder gear as well as the composition of a form disc grinding wheel. By using contour machining principle，a mathematical model of form grinding the involute surface of a spur cylinder gear is established. And also a calculating formula used to dress the profile shape of the transition curve of grinding wheel is given by calculating. This provides a calculation method for dressing the profile shape of disc grinding wheel in the axial section used for the operation of numerical control form grinding spur cylinder gear. The example illustrates the method is effective in the use.