The invention relates to a horn antenna which comprises a horn and a metamaterial sleeve, wherein the metamaterial sleeve is arranged at the opening of the horn and comprises at least one metamaterial ring layer, and the metamaterial ring layer comprises an annular substrate and a plurality of artificial microstructures attached to the surface on the outer side of the annular substrate. The refractive index of any section of each metamaterial ring layer is gradually increased from inside to outside in a direction vertical to the aperture surface of the horn opening. According to the horn antenna, the metamaterial sleeve with the gradually-varied refractive indexes is arranged at the opening of the horn, so that the amplitude distribution of a wavefront electric field is more uniform, therefore the utilization efficiency of the aperture surface of the horn antenna opening is increased, and the far field gain of the horn antenna is increased.