There are inequable force-induced deformation, obvious cutting vibration problems during side milling of low-rigidity frame and beam parts, resulting in unsatisfactory dimensional accuracy and surface quality. Most of these parts are side-milled by end mill, which currently in use do not take into account the workpiece stiffness imparity. Therefore, the concept of gradient helix edge (GHE) is proposed in this research, and its advantages are analyzed. The general mathematical model is given. Furthermore, the parabola is taken as an example for the GHE space equation derived, of which accuracy is verified by numerical simulation. Finally, the comparison experiment with the invariant helix edge end mill was completed. The results showed that the milling force perpendicular to the milling surface of the GHE end mill was smaller and with less fluctuation, which could obtain less force-induced deformation theoretically. The milling process was smoother, and better surface quality was obtained. The excellent performance of the GHE end mill was further verified by milling different features.