Abstract: In order to design a phased array feed with high performance, the relationship between PAF parameters and aperture efficiency is given through the study of optimal sampling of focal field of reflector antenna. The optimal sampling range and element spacing of PAF are summarized, and the calculation formula of element number is derived. The phased array feed of a 9 m antenna is designed. The antenna aperture efficiency is greater than 51% when the scanning range is ±3° in the frequency range of 4−7 GHz, which verifies the effectiveness of the design method. The approach in this paper can be used to guide the engineering design of phased array feed.