A new method of designing continuous phase plates (CPP) for beam smoothing has been studied. The new CPP is applied when the diffraction field is a out-of-focus and curved surface. The traditional Gerchberg-Saxton (GS) algorithm is improved. During algorithm′s each iteration, the out-of-focus plane is regarded as design goal instead of focal plane. And the objective function for compensating the impact of cuned surface is reansomcted. The simulation has proved that the new CPP can control the envelope of focal spot on the out-of-focus curved surface well and obtain high energy concentration, so the demand of the distribution continuity for beam smoothing element, good profile of the target beam and high energy concentration in inertial confinement fusion can be satisfied simultaneously by this algorithm.