The invention discloses a high gain lens antenna with uniform aperture phase and amplitude distribution, and relates to the technical fields of basic electrical elements and communications. The lens antenna includes a metal horn, a dielectric gradient index lens and a metal sheet inserted into the outer wall of the horn in a direction parallel to an electric field. The gradient index lens is disposed at an antenna aperture. The gradient index lens includes two lens portions arranged front and back in a direction of wave propagation. Each lens portion includes two matching layers and a core layer laminated between the two matching layers. Each lens portion has a reflective index varying with one axis direction. The uniform aperture amplitude and phase distribution of the antenna is achieved, and the gain and aperture efficiency of the antenna are improved.