In this paper, the scanning characteristics of an infinite stacked microstrip phased array in a three-layered structure are analysed. In the analysis of the field distribution, the spectral-domain Galerkin Method of Moments together with the planar dyadic Green's function is applied. An attachment mode current is employed to model the singularity of currents nearby the feed point so as to facilitate the fast convergence. The currents on all patches are expanded in terms of the trigonometric basis and weighting functions in the entire domain. The normalized patterns of the infinite microstrip array are computed in this paper and the scanning features of the antenna against the scanning angle and frequency are discussed in both the E- and H-planes.