Abstract: In view of the problem that the delamination was easily occurred during the carbon fiber-reinforced plastic (CFRP) slotting, especially the exit side was the most serious, according to the formation process of the slotting delamination, the onset critical force models of the exit outermost layer delamination for CFRP slotting were emphatically established. The slotting tests, using the T-slot cutter, were carried out on the plain weave CFRP. Then, the formation mechanisms of the delamination were revealed from the view mechanics. The results show that the delaminations of the warp fibers on both up and down sides are small. The onset critical forces of the case A (supported) and case B (unsupported) for the fill fibers are small, then, the delaminations of these two cases easily occur, especially the case B. The critical forces of these two cases are large near the edge of a single fiber bundle, and their changes are small in the central of a single fiber bundle. Additionally, the deformations of these two cases are all large in the center of a single fiber bundle and are small near the edge of a single fiber bundle. As a result, in the boundary of a single bundle of fibers, the delaminations of the case A or case B are all shaped as a half-moon arc “wave peak”. The delaminations of these two cases increase with the increasing feed per tooth f_z and the cutting speed V_c. The experimental results are in good agreement with the theoretical derivation results of the critical forces.