In order to improve focal spot quality in inertial confinement fusion (ICF) system, beam smoothing technology is required so as to meet the strict requirements of target surface irradiated. Continuous phase plate (CPP) can control the shape of focal spot easily and has high diffraction efficiency. In practical optical system, beam smoothing results can be influenced since a variety of factors can cause distorted wavefront of laser beam. The relationship between the full spectrum of distorted wavefront and optical field in the far field is given, and it is obtained that the relationship between 86% focal spot size and the ratio of peak valley value of low frequency distorted wavefront and surface shape of CPP. Meanwhile, the relationship between 86% focal spot size and root-mean-square gradient value of low frequency distorted wavefront is also shown. According to the relationship, the range of controlling distorted wavefront can be found.