Continuous phase plates (CPPs) are important diffractive elements for precise control of beam-shape, energy distribution, and wavefront profile in large laser assembly. A theoretical design model for CPP used in inertial confinement fusion is built to improve the focus beam quality and reduce the difficulty of CPP′s fabrication. Moreover, the traditional G-S algorithm is improved through choosing continuous initial phase, phase unwrapping, phase filtering and controlling the frequency spectrum of focal spot. The operational characteristics and the focal spot′s performance caused by the CPP designed by traditional and improved G-S algorithm are analyzed comparatively. The CPP with 330 mm of diameters used for back lighting is designed and first fabricated using numerical control wet-etching. It shows that the CPP designed by the improved method meets the process restriction and physical specifications very well.