Abstract: A method of calculating focal field is introduced, and the connection between focal field, which depends on the reflector geometry, and the aperture field of feed is analyzed. Based on focal field distribution, a design method of phased array feed, including element spacing determination, phased array scale, and excitations for expected radiation pattern is presented. Two phased array feeds are designed to realize shaped beams for an un-shaped Cassegrain antenna and shaped scanning beams for a prime-focus parabolic antenna respectively. The numerical analyses demonstrate the effectiveness of the approach.