The invention provides a low-RCS phased-array antenna and an RCS reduction method thereof. Each antenna unit comprises four layers of structures, namely, a radiation patch layer, a cross-shaped feed structure layer, a floor layer and a direct-current feed layer, and every two adjacent layers are separated through a dielectric substrate. Switching among antenna circular polarization modes and switching among antenna feed points are achieved by controlling on-off of PIN diodes, so that the effect of equivalent rotation of the antenna units in the horizontal direction is achieved, and a phase difference of 180 degrees, which is twice of the rotation angle, of the scattering fields of the antenna units is achieved. The scattering fields of the antenna units are counteracted in a far field anddispersed to a wider angular domain space, so that the RCS of the array antenna is reduced. The scattering phases of the antenna units are directly controlled, so that the scattering fields of the array elements cannot be coherently superposed or reflected to a threat-free angle, and the RCS of the array is reduced.