In order to satisfy uniform irradiation requirement in inertial confinement fusion(ICF) system, the influence of surface characteristics of continuous phase plate on beam quality is studied in this paper. Based on wave optics and geometrical optics, theoretical analysis model is established, respectively. And the parameter, root mean square gradient, is used to unify both theoretical models. Meanwhile, the smoothing effects of root mean square phase gradient of continuous phase plate on laser beam with low-frequency aberrant wavefront are simulated. Numerical simulations show that the radius of focus spot will increase with the increase of root mean square phase gradient of continuous phase plate, and nonuniformity of beam top will decrease quickly, then reduce slowly and tend to be unchanged finally. So smoothing effect of continuous phase plate is obvious. In addition, energy availability factor has little change, then decreases gradually and reduces slowly. Spot size, nonuniformity of beam top and energy availability factor are all in a good range when the correlation length of continuous phase plate is not changed and root mean square gradient is 0.20.8 wave/mm.