This paper presents the preliminary simulation results of the prototype 40-element phased array feed (PAF) array for the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and the measurement of a manufactured single Vivaldi element. The simulation results include the characteristic of a standalone Vivaldi element. The full-wave simulation of the explicit PAF array is carried out, from which the performance of the embedded elements and the mutual coupling among the elements could be accurately inferred. The performance of the combination of the PAF and the main reflector of FAST is calculated. The results imply that the PAF combined with the FAST antenna could operate over a much wider bandwidth from 650 to 1450 MHz compared with the standalone element. The antenna temperature remains almost the same for zenith angles from 0 to 40°. From the simulation results, the PAF shows good potential to improve the performance of FAST. A measurement of a manufactured Vivaldi element is also presented.