We design a kind of continuous phase plate (CPP) used in high power laser driver to improve the focus beam quality. The optimal design based on controlling segmented spatial spectrum is modified from traditional G-S algorithm. The improved algorithm chooses a restrained initial phase that is related with the low frequency information of goal focal spot’s spatial spectrum, and the medium frequency and high frequency can be appropriately controlled in iterations. The experimental results show that the root mean square error (RMS) of the focal spot can quickly converge to the target threshold under the effect of improved CPP, which compared with that under the effect of the traditional random continuous initial phase. The improvement of initial phase not only can avoids the G-S algorithm easily produces local optimum value, but also the improved method has a good application for the design of asymmetric focal spot.