The splitting extrusion of dovetail groove and the cleaving wedge were introduced. Aimed at the phenomenon of notch expanding during dovetail groove forming process, four kinds of splitting edge inclinations were took from the range of +30° to -30°. At the same time, the fixed splitting angle was 60°. By means of numerical simulation test, the result shows that the forming sequence of the slot cross section is determined by the ridge surfaces inclination. The main reason of notch expanding is the positive ridge surface inclination which is confirmed by physical simulation testing. To ensure the strength of cleaving wedge, it is suggested that the ridge surface inclination to be 0° and the manufacturing error has to be eliminated by deviating the inclination towards negative side.