During the milling of the ball-end milling cutter in the splicing area of the curved surface mold, the tool load changes greatly, and the transient milling force has a sudden change, which affects the machining accuracy and surface quality of the splicing area of the mold. In light of the prediction of its transient milling force in the splicing area, a transient undeformed chip thickness model is established based on a three-dimensional trochoid trajectory equation of the ball-end milling cutter considering impact vibration. Then, based on the idea of milling microelement, the transient milling force model for ball-end milling cutter of the convex curved surface and double hardness stitching mold is established. The model comprehensively considers the influence of impact vibration, the change of hardness and contact angle of the tool workpiece in the splicing area on the transient milling force. Finally, the milling experiments of ball-end milling cutter in the splicing area of convex surface is carried out. The results show that the predicted transient milling force and experimental measurements are consistent in magnitude and trend. The maximum milling force prediction error value is less than 15% during steady cutting. The experimental results verify that the model can effectively predict the transient milling force of the ball-end milling cutter in the splicing area of the convex surface of the splicing mold.