Abstract: When plane wave illuminates a infinite periodic array,periodic current will be induced on it.Assuming the finite array is a part of an infinite array,then the Green function of the finite array can be obtained by windowing the Green function of the infinite array,thus the analysis of the finite array can be reduced to that of a single element.Moment method is used to calculate the element current and scattered field,and we define this scattered field as the Active Element Scattering Pattern(AESP).According to the pattern synthesis theory,the scattering of the whole array is the multiplication of the AESP and an array factor,so the analysis of electrically large finite array is greatly simplified.Numerical results are presented to show the validity of the approach.