As a force being non-homogenous during incremental forming process, sheet metal tended to instability easily, then the deposition, wrinkle and fracture will be appearing concomitantly. If the vibration technique is introduced to the incremental forming process, its deformation mechanism will be changed, and forming quality is improved. Then some force equations are built by force analyzed for a element in local forming contact zone, some solutions of the force equations are obtained. With process parameters, mechanical performance parameters and vibra parameters given, stress curves are got. It is shown from simulation that the sheet metal stress has prominent variation with vibration，and forming-angle condition and frequency-condition should be met with vibration.