Aiming at the problems of beam scanning reflectarray antennas such as gain reduction and pattern deterioration during large-angle scanning, a method is proposed to improve the scanning performance of the reflectarray by defocusing the feed. Without loss of generality, a reflectarray antenna that works in the X-band is designed with a square patch structure. The effects of longitudinal defocusing at 15°, 30°, and 45°scanning angles and lateral defocusing at 10°, 20°, and 30°scanning angles on the performance of beam scanning reflectarray antennas are studied. The measured results show that when the scanning angle is small, the lateral defocus causes the beam direction to change, and the beamwidth does not change much. When reducing the size of the longitudinal defocus of the feed, the sidelobe level is reduced, and the gain at the center frequency is improved. The larger the scanning angle is, the more obvious the effect becomes. This is beneficial to improve the large-angle scanning capability of the beam scanning reflective array antenna.