Abstract: In order to determine the position of equivalent phase centers of pyramidal horn antenna, which keep the phase of the antenna's field relative invariable in some range of the antenna's main lobe, an approach to analyze the equivalent phase centers of pyramidal horn antenna is presented in this paper. Through analyzing the far field of the pyramidal horn using its simplified model, the E and H plane phase centers and their equiphase surface width of the pyramidal horn are analyzed and calculated by using the method of moving reference point, and the relationship of deviation of equiphase surface between the phase center and equiphase surface width is achieved.